Adding a whole extra header file just for this is not the optimal
solution... but stuffing it in with anything else exposes things that
otherwise wouldn't be.
Nothing in userland should use <sys/semaphore.h> directly, and if some
foolish third-party software should decide to do so anyway in spite of
the instructions to the contrary I will ruthlessly break it later when
the big kernel includes cleanup finally happens.
Reported by Kamil Rytarowski, and, as it turns out, also by Klaus
Heinz in 2008.
521eaa499b