contrib: add GPL license tag on files that miss it

Some file miss completely the license tag.

Add the SPDX tag, using the same GPL-2.0-or-later license of the
OpenOCD project.

The SPDX tag on files *.c is incorrect, as it should use the C99
single line comment using '//'. But current checkpatch doesn't
allow C99 comments, so keep using standard C comments, by now.

Change-Id: I24bd362eeb6b74f09aceb9b757d45cbfa4afe334
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7160
Tested-by: jenkins