[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.

0 files changed, 0 insertions, 0 deletions