compositor-fbdev: unref udev on backend destruction

Fixes a small memory leak, spotted with Valgrind.

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

libweston/compositor-fbdev.c

@@ -665,6 +665,8 @@ fbdev_backend_destroy(struct weston_compositor *base)
 	/* Chain up. */
 	weston_launcher_destroy(base->launcher);
 
+	udev_unref(backend->udev);
+
 	free(backend);
 }