selftests: kselftest_install.sh: 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/kselftest_install.sh

 #!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
 #
 # Kselftest Install
 # Install kselftest tests
 # 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).
-
 install_loc=`pwd`
 
 main()