Try fix macOS build with picojson (mlc-ai#1206)

The error message below

```
/usr/share/miniconda/envs/mlc-llm-build/conda-bld/mlc-chat-cli-nightly-package_1699286394016/work/3rdparty/tvm/3rdparty/picojson/picojson.h: In member function 'std::string picojson::value::to_str() const':
/usr/share/miniconda/envs/mlc-llm-build/conda-bld/mlc-chat-cli-nightly-package_1699286394016/work/3rdparty/tvm/3rdparty/picojson/picojson.h:494:37: error: expected ')' before 'PRId64'
  494 |       SNPRINTF(buf, sizeof(buf), "%" PRId64, u_.int64_);
      |               ~                     ^~~~~~~
      |                                     )
/usr/share/miniconda/envs/mlc-llm-build/conda-bld/mlc-chat-cli-nightly-package_1699286394016/work/3rdparty/tvm/3rdparty/picojson/picojson.h:81:1: note: 'PRId64' is defined in header '<cinttypes>'; did you forget to '#include <cinttypes>'?
   80 | #include <errno.h>
  +++ |+#include <cinttypes>
   81 | #include <inttypes.h>

```

indicates that the `__STDC_FORMAT_MACROS` flag is not turned on for some
reason.

Author: junrushao

cpp/json_parser.h

@@ -2,9 +2,7 @@
 #define MLC_LLM_CPP_JSON_PARSER_H_
 
 #define PICOJSON_USE_INT64
-#ifndef __STDC_FORMAT_MACROS
 #define __STDC_FORMAT_MACROS
-#endif
 
 #include <picojson.h>
 #include <tvm/runtime/container/shape_tuple.h>