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>

tools/testing/selftests/resctrl/Makefile

+# SPDX-License-Identifier: GPL-2.0
+
 CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2
 CFLAGS += $(KHDR_INCLUDES)