futex: move required includes to header

lkmc/futex.h

@@ -1,6 +1,13 @@
 #ifndef LKMC_FUTEX_H
 #define LKMC_FUTEX_H
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#include <linux/futex.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
 static int
 lkmc_futex(int *uaddr, int futex_op, int val,
         const struct timespec *timeout, int *uaddr2, int val3)

userland/linux/futex.c

@@ -1,13 +1,10 @@
 /* https://cirosantilli.com/linux-kernel-module-cheat#futex-system-call */
 
-#define _GNU_SOURCE
 #include <assert.h>
-#include <linux/futex.h>
 #include <pthread.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
-#include <sys/syscall.h>
 
 #include <lkmc/futex.h>