[llvm] Fix incorrect usage of LIBXML2_INCLUDE_DIRS in the Windows release script (#95781)

Before this fix, when building the Windows LLVM package with the latest
cmake 3.29.3 I was seeing:
```
C:\git\llvm-project>llvm\utils\release\build_llvm_release.bat --version 19.0.0 --x64 --skip-checkout --local-python
...
-- Looking for FE_INEXACT
-- Looking for FE_INEXACT - found
-- Performing Test HAVE_BUILTIN_THREAD_POINTER
-- Performing Test HAVE_BUILTIN_THREAD_POINTER - Failed
-- Looking for mach/mach.h
-- Looking for mach/mach.h - not found
-- Looking for CrashReporterClient.h
-- Looking for CrashReporterClient.h - not found
-- Looking for pfm_initialize in pfm
-- Looking for pfm_initialize in pfm - not found
-- Could NOT find ZLIB (missing: ZLIB_LIBRARY ZLIB_INCLUDE_DIR)
CMake Error at C:/Program Files/CMake/share/cmake-3.29/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find LibXml2 (missing: LIBXML2_INCLUDE_DIR)
Call Stack (most recent call first):
  C:/Program Files/CMake/share/cmake-3.29/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
  C:/Program Files/CMake/share/cmake-3.29/Modules/FindLibXml2.cmake:108 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  cmake/config-ix.cmake:167 (find_package)
  CMakeLists.txt:921 (include)


-- Configuring incomplete, errors occurred!
```
It looks like `LIBXML2_INCLUDE_DIRS` (with the extra 'S') is a result
variable that is set by cmake after a call to `find_package(LibXml2)`.
It is actually `LIBXML2_INCLUDE_DIR` (without the 'S') that shold be
used as a input before the `find_package` call, since the 'S' variable
is unconditionally overwritten, see
https://github.com/Kitware/CMake/blob/master/Modules/FindLibXml2.cmake#L96.
I am unsure exactly why that worked with older cmake versions.

Author: aganea

llvm/utils/release/build_llvm_release.bat

@@ -194,7 +194,7 @@ set cmake_flags=^
   -DLLDB_TEST_COMPILER=%stage0_bin_dir%/clang.exe ^
   -DPYTHON_HOME=%PYTHONHOME% ^
   -DPython3_ROOT_DIR=%PYTHONHOME% ^
-  -DLIBXML2_INCLUDE_DIRS=%libxmldir%/include/libxml2 ^
+  -DLIBXML2_INCLUDE_DIR=%libxmldir%/include/libxml2 ^
   -DLIBXML2_LIBRARIES=%libxmldir%/lib/libxml2s.lib
 
 cmake -GNinja %cmake_flags% %llvm_src%\llvm || exit /b 1
@@ -250,7 +250,7 @@ set cmake_flags=^
   -DLLDB_TEST_COMPILER=%stage0_bin_dir%/clang.exe ^
   -DPYTHON_HOME=%PYTHONHOME% ^
   -DPython3_ROOT_DIR=%PYTHONHOME% ^
-  -DLIBXML2_INCLUDE_DIRS=%libxmldir%/include/libxml2 ^
+  -DLIBXML2_INCLUDE_DIR=%libxmldir%/include/libxml2 ^
   -DLIBXML2_LIBRARIES=%libxmldir%/lib/libxml2s.lib
 
 cmake -GNinja %cmake_flags% %llvm_src%\llvm || exit /b 1
@@ -317,7 +317,7 @@ set "stage0_bin_dir=%build_dir%/build_arm64_stage0/bin"
 set cmake_flags=^
   %common_cmake_flags% ^
   -DCLANG_DEFAULT_LINKER=lld ^
-  -DLIBXML2_INCLUDE_DIRS=%libxmldir%/include/libxml2 ^
+  -DLIBXML2_INCLUDE_DIR=%libxmldir%/include/libxml2 ^
   -DLIBXML2_LIBRARIES=%libxmldir%/lib/libxml2s.lib ^
   -DPython3_ROOT_DIR=%PYTHONHOME% ^
   -DCOMPILER_RT_BUILD_PROFILE=OFF ^