compositor-drm: unref udev on backend destruction

Fixes a small memory leak, spotted with Valgrind.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

libweston/compositor-drm.c

@@ -3600,6 +3600,8 @@ drm_destroy(struct weston_compositor *ec)
 	if (b->gbm)
 		gbm_device_destroy(b->gbm);
 
+	udev_unref(b->udev);
+
 	weston_launcher_destroy(ec->launcher);
 
 	close(b->drm.fd);