gettime-thread: fix missing startup mutex

It'll segfault if gtod_cpu= is used, ensure we pass in the mutex.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/gettime-thread.c b/gettime-thread.c
index 72cc4d8..95f35c3 100644
--- a/gettime-thread.c
+++ b/gettime-thread.c
@@ -63,7 +63,7 @@
 
 	pthread_attr_init(&attr);
 	pthread_attr_setstacksize(&attr, PTHREAD_STACK_MIN);
-	ret = pthread_create(&gtod_thread, &attr, gtod_thread_main, NULL);
+	ret = pthread_create(&gtod_thread, &attr, gtod_thread_main, mutex);
 	pthread_attr_destroy(&attr);
 	if (ret) {
 		log_err("Can't create gtod thread: %s\n", strerror(ret));