Use g_autoptr() for GIO object types

GLib 2.44 supplies all the necessary autocleanup macros for GIO types,
and libglnx backports the relevant macros for ostree.

src/libostree/ostree-metalink.c

@@ -432,7 +432,7 @@ on_fetched_url (GObject              *src,
   GError *local_error = NULL;
   struct stat stbuf;
   int parent_dfd = _ostree_fetcher_get_dfd (self->metalink->fetcher);
-  gs_unref_object GInputStream *instream = NULL;
+  g_autoptr(GInputStream) instream = NULL;
   g_autofree char *result = NULL;
   GChecksum *checksum = NULL;