failtest: include config.h first

It's now important, since it defines _GNU_SOURCE.

ccan/failtest/failtest.h

 #ifndef CCAN_FAILTEST_H
 #define CCAN_FAILTEST_H
+#include "config.h"
 #include <sys/types.h>
 #include <stdbool.h>
 #include <fcntl.h>

ccan/failtest/failtest_override.h

 #ifndef CCAN_FAILTEST_OVERRIDE_H
 #define CCAN_FAILTEST_OVERRIDE_H
 /* This file is included before the source file to test. */
+#include "config.h"
 
 /* Replacement of allocators. */
 #include <stdlib.h>