[openmp] Fix missing include directory in omptest tool (#156194)

Add missing `LIBOMP_INCLUDE_DIR` include directory to fix build failures
in omptest, as reported
in
https://github.com/llvm/llvm-project/pull/154786#issuecomment-3223481804.
Thanks fo @jprotze for the suggested fix.

Signed-off-by: Michał Górny <mgorny@gentoo.org>
GitOrigin-RevId: 1d8fdda7b0a9f47c443600bca6af2bc141e4abf7