compositor-drm: free filename in exit

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

@@ -5229,6 +5229,7 @@ drm_destroy(struct weston_compositor *ec)
 	wl_array_release(&b->unused_connectors);
 
 	close(b->drm.fd);
+	free(b->drm.filename);
 	free(b);
 }