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>
* Causes a lockup on SMP systems Reviewed-by: Alexander von Gluck IV <kallisti5@unixzen.com> Part-of: <!8323>