pipewire-jack: do not dereference value from `pw_data_loop_get_thread()`

`pw_data_loop_get_thread()` returns a `pthread_t` cast to `struct spa_thread *`,
so simply casting it back to `pthread_t` is enough.

pipewire-jack/src/pipewire-jack.c

@@ -3777,7 +3777,7 @@ jack_native_thread_t jack_client_thread_id (jack_client_t *client)
 	thr = pw_data_loop_get_thread(c->loop);
 	if (thr == NULL)
 		return pthread_self();
-	return *(pthread_t*)thr;
+	return (pthread_t) thr;
 }
 
 SPA_EXPORT