Fix clang warning in gguf_check_reserved_keys

[yeahdongcn]

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We recently enabled our in-house CI for MUSA backend, and this warning was detected in a fresh build:

[  0%] Building CXX object ggml/src/CMakeFiles/ggml-base.dir/gguf.cpp.o
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = unsigned char; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:944:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
  930 | static void gguf_check_reserved_keys(const std::string & key, const T val) {
      |                                                               ~~~~~~~~^~~
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = signed char; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:950:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = short unsigned int; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:956:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = short int; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:962:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = int; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:974:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = float; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:980:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = long unsigned int; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:986:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = long int; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:992:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = double; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:998:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = bool; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:1004:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = const char*; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:1010:38:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = const void*; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:1016:39:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp: In instantiation of ‘void gguf_check_reserved_keys(const string&, T) [with T = const char**; std::string = std::__cxx11::basic_string<char>]’:
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:1029:39:   required from here
/home/xiaodongye/ws/ggml/llama.cpp/ggml/src/gguf.cpp:930:71: warning: parameter ‘val’ set but not used [-Wunused-but-set-parameter]