build: Add missing "static" keyword where it should be used

Otherwise we fail to build with -Werror=missing-prototypes.

https://bugzilla.gnome.org/show_bug.cgi?id=687385

gio/gdbusactiongroup.c

@@ -92,7 +92,7 @@ action_info_free (gpointer user_data)
   g_slice_free (ActionInfo, info);
 }
 
-ActionInfo *
+static ActionInfo *
 action_info_new_from_iter (GVariantIter *iter)
 {
   const gchar *param_str;