xray: Add missing header to list of installed headers. Spotted by gn's llvm/utils/gn/build/sync_source_lists_from_cmake.py script. Differential Revision: https://reviews.llvm.org/D56797 llvm-svn: 351479 GitOrigin-RevId: 48807d0d35638911e9dc65ae793234f2f3f582cb