Add include directory for libxml on macOS

Summary:
Builds fail because libxml/xmlreader.h isn't found. Adding the include
directory to the include list fixes the issue. This is what we already do on
non-macOS platforms in the same file.

Reviewers: clayborg, xiaobai, lanza

Reviewed By: clayborg, lanza

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D50918

llvm-svn: 340460