girepository: Add SPDX license header to all obvious files

Add the SPDX license runes to all the files which have an obvious
copyright header already. This is a mechanical edit.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Helps: #3155

girepository/gistructinfo.c

@@ -4,6 +4,8 @@
  * Copyright (C) 2005 Matthias Clasen
  * Copyright (C) 2008,2009 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