TT#83954 Invoke libevent_global_shutdown() only conditionally

Commit a6d2f9296 introduced usage of libevent_global_shutdown(),
which is available in libevent-dev >=2.1.1-alpha only.

For example travis CI is running Ubuntu 16.04 / xenial and ships
libevent-dev 2.0.21-stable-2, so we can't just raise the
build-dependency of libevent-dev to >= 2.1.1+ if we still want
to support older systems.

Change-Id: I9dcf9ca0a19f59051d42f0d2383d1938b60a97df

daemon/redis.c

@@ -632,7 +632,9 @@ void redis_notify_loop(void *d) {
 	}
 
 	// free libevent
+#if LIBEVENT_VERSION_NUMBER >= 0x02010100
 	libevent_global_shutdown();
+#endif
 
 	// unsubscribe notifications
 	redis_notify_subscribe_action(UNSUBSCRIBE_ALL, 0);