selftests/resctrl: Move _GNU_SOURCE define into Makefile

_GNU_SOURCE is defined in resctrl.h. Defining _GNU_SOURCE has a large
impact on what gets defined when including headers either before or
after it. This can result in compile failures if .c file decides to
include a standard header file before resctrl.h.

It is safer to define _GNU_SOURCE in Makefile so it is always defined
regardless of in which order includes are done.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Cc: <stable@vger.kernel.org>
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 = -g -Wall -O2 -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE
 CFLAGS += $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS := resctrl_tests

tools/testing/selftests/resctrl/resctrl.h

 /* SPDX-License-Identifier: GPL-2.0 */
-#define _GNU_SOURCE
 #ifndef RESCTRL_H
 #define RESCTRL_H
 #include <stdio.h>