utils: Add missing SPDX headers to scripts

The gen-header.sh and gen-shader-headers.sh scripts are missing an SPDX
header. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Bryan O'Donoghue <bod.linux@nxsw.ie>

utils/codegen/gen-header.sh

@@ -1,4 +1,5 @@
 #!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-or-later
 
 dst_file="$1"
 shift

utils/gen-shader-headers.sh

@@ -1,4 +1,5 @@
 #!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-or-later
 
 set -e