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 <[e-mail]>
Reviewed-by: Fenghua Yu <[e-mail]>
Signed-off-by: Shaopeng Tan <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/resctrl/Makefile

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