openocd: src/helper: replace the GPL-2.0-or-later license tag

openocd: src/helper: replace the GPL-2.0-or-later license tag

Replace the FSF boilerplate with the SPDX tag.

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: I7851617e2682f97ccc3927e3941aadef2df63b54
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7068
Tested-by: jenkins