codegen: Trivially add SPDX-License-Identifier to codegen Python files

The license and copyright are already stated in human-readable form in
these files, so this should be uncontroversial.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #1415

gio/gdbus-2.0/codegen/parser.py

@@ -4,6 +4,8 @@
 #
 # Copyright (C) 2008-2011 Red Hat, Inc.
 #
+# SPDX-License-Identifier: LGPL-2.1-or-later
+#
 # This library is free software; you can redistribute it and/or
 # modify it under the terms of the GNU Lesser General Public
 # License as published by the Free Software Foundation; either