selftests: gen_kselftest_tar.h: Add SPDX license identifier

Replace GPL license statement with SPDX GPL-2.0 license identifier.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/gen_kselftest_tar.sh

 #!/bin/bash
 #
+# SPDX-License-Identifier: GPL-2.0
 # gen_kselftest_tar
 # Generate kselftest tarball
 # Author: Shuah Khan <shuahkh@osg.samsung.com>
 # Copyright (C) 2015 Samsung Electronics Co., Ltd.
 
-# This software may be freely redistributed under the terms of the GNU
-# General Public License (GPLv2).
-
 # main
 main()
 {