uncrustify.cfg: add SPDX license

Add the SPDX tag line.

Change-Id: I701580948a0cacdb7fe31d91ed730e848da9b0ba
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8411
Reviewed-by: zapb <dev@zapb.de>
Tested-by: jenkins

uncrustify.cfg

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
 tok_split_gte=false
 utf8_byte=false
 utf8_force=false