[OpenMP] Fixed an issue that wrong LLVM headers might be included when building libomptarget

Wrong LLVM headers might be included if we don't set `include_directories`
to a right place. This will cause a compilation error if LLVM is installed in
system directories.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93737

GitOrigin-RevId: 5acdae1f9a772e16850b0ee9648d79a7c3becf23
1 file changed