openocd: src: fix incorrect GPL license tag

openocd: src: fix incorrect GPL license tag

Use the standard SPDX tag, where it was incorrectly applied.

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: Iaec63abc6e0a38e5b0ae0ea7f5ecee7ca007bbbd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7065
Tested-by: jenkins