[OpenMP] Fix SystemZ build failure

Commit a7d5f73a03c81cab8df64dbd099e8acb40f5dfe1 introduced an
error in a target_compile_definitions on the SystemZ, causing
the build to break.  Fixed by adding the missing "PRIVATE".

GitOrigin-RevId: cb071942f881e743b8131688a873dab760c7b88d
1 file changed