Log Message:
Remove incorrect patch.

Upstream (Bruno Haible) write:
The comment says that this include of <pthread.h> is there to make sure that
<signal.h> declares pthread_sigmask.

But on DragonFly (I checked versions 3.8, 4.6, and 4.8.0-RC), pthread_sigmask
is already declared in <signal.h>, not in <pthread.h>.