unbreak building kernel without option INET6

Reported by:	markp

sys/net/if_faith.c

@@ -292,6 +292,7 @@ faithioctl(ifp, cmd, data)
 	return (error);
 }
 
+#ifdef INET6
 /*
  * XXX could be slow
  * XXX could be layer violation to call sys/net from sys/netinet6
@@ -321,4 +322,5 @@ faithprefix(in6)
 		RTFREE(rt);
 	return ret;
 }
+#endif
 #endif /* NFAITH > 0 */

sys/net/if_faith.h

@@ -34,8 +34,10 @@
 #define _NET_IF_FAITH_H_
 
 #ifdef _KERNEL
+#ifdef INET6
 struct in6_addr;
 int faithprefix __P((struct in6_addr *));
 #endif
+#endif
 
 #endif /* _NET_IF_FAITH_H_ */