[CMAKE] Honor CMAKE_OSX_SYSROOT to compute include dir for libxml2

On MacOS, if CMAKE_OSX_SYSROOT is used and the user has command line tools
installed, we currently get the include path for libxml2 as
/usr/include/libxml2, instead of ${CMAKE_OSX_SYSROOT}/usr/include/libxml2.

Make it consistent on MacOS by prefixing ${CMAKE_OSX_SYSROOT} when
possible.

rdar://problem/41103601

llvm-svn: 334746