util/u_thread: Disable pthread_barrier_t on Haiku

* Causes a lockup on SMP systems

Reviewed-by: Alexander von Gluck IV <kallisti5@unixzen.com>
Part-of: <!8323>
73 jobs for !8323 with jan2021-haiku-fixes in 19 minutes and 3 seconds (queued for 5 seconds)
merge request