WireGuardKitC: fix build with Xcode 16b1

https://github.com/WireGuard/wireguard-apple/pull/33/files

Author: Waldz

Sources/WireGuardKitC/WireGuardKitC.h

@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: MIT
 // Copyright © 2018-2023 WireGuard LLC. All Rights Reserved.
 
+#include <sys/types.h>
+
 #include "key.h"
 #include "x25519.h"