LibC: Properly implement the futimens function

Use the new futimens syscall to ensure futimens can actually work.
This change for example allows a user to run "touch non-existing-file"
without getting any error, as expected.

Userland/Libraries/LibC/bits/utimens.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2018-2022, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, sin-ack <sin-ack@protonmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <Kernel/API/POSIX/fcntl.h>
+#include <Kernel/API/POSIX/sys/stat.h>
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+int __utimens(int fd, char const* path, struct timespec const times[2], int flag);
+
+__END_DECLS