fix a re-typedef caused by libdnet update

ok pvalchev@

security/fragroute/patches/patch-tun_h

@@ -0,0 +1,13 @@
+$OpenBSD: patch-tun_h,v 1.1 2005/05/22 05:39:26 mjc Exp $
+--- tun.h.orig	Sat May 21 21:35:11 2005
++++ tun.h	Sat May 21 21:35:34 2005
+@@ -9,7 +9,9 @@
+ #ifndef TUN_H
+ #define TUN_H
+ 
++#ifndef DNET_TUN_H
+ typedef struct tun tun_t;
++#endif
+ 
+ typedef void (*tun_handler)(void *buf, size_t len, void *arg);
+