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>
Reviewed-by: Ian Ray <ian.ray@ge.com>
diff --git a/libweston/compositor-drm.c b/libweston/compositor-drm.c
index 578ff9d..5e87cb4 100644
--- a/libweston/compositor-drm.c
+++ b/libweston/compositor-drm.c
@@ -3582,6 +3582,8 @@
 	if (b->gbm)
 		gbm_device_destroy(b->gbm);
 
+	udev_unref(b->udev);
+
 	weston_launcher_destroy(ec->launcher);
 
 	close(b->drm.fd);