Add SPDX license headers for LGPL-2.1-or-later to various files

These have all been added manually, as I’ve finished all the files which
I can automatically detect.

All the license headers in this commit are for LGPL-2.1-or-later, and
all have been double-checked against the license paragraph in the file
header.

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

Helps: #1415

gio/tests/gtlsconsoleinteraction.h

@@ -2,6 +2,8 @@
  *
  * Copyright (C) 2011 Collabora, Ltd.
  *
+ * 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