Prefork pool must always enable timeouts as they may be set at runtime.

Closes  celery#3174

celery/concurrency/prefork.py

@@ -112,6 +112,7 @@ def on_start(self):
         P = self._pool = Pool(processes=self.limit,
                               initializer=process_initializer,
                               on_process_exit=process_destructor,
+                              enable_timeouts=True,
                               synack=False,
                               **self.options)