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
1 file changed