selftests/resctrl: Add missing SPDX license to Makefile

[ Upstream commit 68c48449 ]

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>
Stable-dep-of: 3a1e4a91 ("selftests/resctrl: Move _GNU_SOURCE define into Makefile")
Signed-off-by: Sasha Levin <sashal@kernel.org>