[CMake] Use LIBXML2_INCLUDE_DIR instead of hardcoding /usr/include/libxml2

`find_package(LibXml2 REQUIRED)` sets `LIBXML2_INCLUDE_DIR` to the correct
include directory for the libxml2 headers. Use this variable instead of
hardcoding `/usr/include/libxml2`. This allows the build to work with
custom libxml2 builds on WASI.

Author: kateinoigakukun

Sources/_CFXMLInterface/CMakeLists.txt

@@ -20,7 +20,7 @@ target_include_directories(_CFXMLInterface
         ../CoreFoundation/include
     PRIVATE
         ../CoreFoundation/internalInclude
-        /usr/include/libxml2/)
+        ${LIBXML2_INCLUDE_DIR})
 
 target_compile_options(_CFXMLInterface INTERFACE
   "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xcc -fmodule-map-file=${CMAKE_CURRENT_SOURCE_DIR}/../CoreFoundation/include/module.modulemap>"