fix compile error

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@266240 65c4cc65-6c06-0410-ace0-fbb531ad65f3

pbx/pbx_realtime.c

@@ -136,6 +136,8 @@ static void *cleanup(void *unused)
 		pthread_testcancel();
 		nanosleep(&one_second, NULL);
 	}
+
+	return NULL;
 }