tcl: move SPDX tag as first line

tcl: move SPDX tag as first line

The SPDX tag is aimed at machine handling and it's thus expected
to be placed in the first line.

Change-Id: I3992856eeb28b333c38d010ef286e22471ede263
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7026
Tested-by: jenkins