selftests/resctrl: Add missing SPDX license to Makefile

Add the missing SPDX(SPDX-License-Identifier) license header to
tools/testing/selftests/resctrl/Makefile.

Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>