[gn build] Don't copy xray includes

The gn build doesn't support xray, so there's no reason to make the xray
headers available. Some CMake checks check if xray includes are
available to determine if xray is usable. Since we don't build the xray
runtime, there are link errors.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D108737