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>
diff --git a/libweston/compositor-fbdev.c b/libweston/compositor-fbdev.c
index 10229be..62ffa0b 100644
--- a/libweston/compositor-fbdev.c
+++ b/libweston/compositor-fbdev.c
@@ -665,6 +665,8 @@
 	/* Chain up. */
 	weston_launcher_destroy(base->launcher);
 
+	udev_unref(backend->udev);
+
 	free(backend);
 }