compositor-drm: unref udev monitor on exit

Leaks spotted by Valgrind.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>

libweston/compositor-drm.c

@@ -5221,6 +5221,7 @@ drm_destroy(struct weston_compositor *ec)
 	if (b->gbm)
 		gbm_device_destroy(b->gbm);
 
+	udev_monitor_unref(b->udev_monitor);
 	udev_unref(b->udev);
 
 	weston_launcher_destroy(ec->launcher);