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

gthread/gthread-impl.c

@@ -1,6 +1,8 @@
 /* GLIB - Library of useful routines for C programming
  * Copyright (C) 1995-1997  Peter Mattis, Spencer Kimball and Josh MacDonald
  *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
  * gthread.c: thread related functions
  * Copyright 1998 Sebastian Wilhelmi; University of Karlsruhe
  *