c11: Do not use pthread_mutex_timedlock on NetBSD.

This patch fixes the NetBSD build.

NetBSD does not have pthread_mutex_timedlock.

  CC       glapi_dispatch.lo
threads_posix.h: In function 'mtx_timedlock':
threads_posix.h:216:5: error: implicit declaration of function 'pthread_mutex_timedlock'

Signed-off-by: Vinson Lee <vlee@freedesktop.org>