[fix] missing _GNU_SOURCE definition in places

Add config.h to files that depend on _GNU_SOURCE for
symbols.

See #44.

src/area/hwloc.c

@@ -8,6 +8,8 @@
  * SPDX-License-Identifier: BSD-3-Clause
  ******************************************************************************/
 
+#include "config.h"
+
 #include <sys/mman.h>
 
 #include "aml.h"

tests/area/test_hwloc.c

@@ -7,9 +7,11 @@
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *******************************************************************************/
+#include "config.h"
 
 #include <assert.h>
 #include <hwloc.h>
+#include <stdlib.h>
 
 #include "aml.h"