[llvm] Fix typo for libxml2 detection

This seems to be a copy-paste from the similar zlib detection code.

Patch By: sthibaul

Differential Revision: https://reviews.llvm.org/D117052

llvm/lib/WindowsManifest/CMakeLists.txt

@@ -26,7 +26,7 @@ if(LLVM_ENABLE_LIBXML2)
     string(TOUPPER ${CMAKE_BUILD_TYPE} build_type)
     get_property(libxml2_library TARGET LibXml2::LibXml2 PROPERTY LOCATION_${build_type})
   endif()
-  if(NOT zlib_library)
+  if(NOT libxml2_library)
     get_property(libxml2_library TARGET LibXml2::LibXml2 PROPERTY LOCATION)
   endif()
   get_library_name(${libxml2_library} libxml2_library)