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-kernel-args.c

@@ -163,7 +163,7 @@ _ostree_kernel_args_append_proc_cmdline (OstreeKernelArgs *kargs,
                                          GCancellable     *cancellable,
                                          GError          **error)
 {
-  gs_unref_object GFile *proc_cmdline_path = g_file_new_for_path ("/proc/cmdline");
+  g_autoptr(GFile) proc_cmdline_path = g_file_new_for_path ("/proc/cmdline");
   g_autofree char *proc_cmdline = NULL;
   gsize proc_cmdline_len = 0;
   gs_strfreev char **proc_cmdline_args = NULL;