[IPV6] kill a warning when building without CONFIG_SYSCTL.

Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>

net/ipv6/addrconf.c

@@ -404,6 +404,7 @@ static struct inet6_dev * ipv6_find_idev(struct net_device *dev)
 	return idev;
 }
 
+#ifdef CONFIG_SYSCTL
 static void dev_forward_change(struct inet6_dev *idev)
 {
 	struct net_device *dev;
@@ -449,7 +450,7 @@ static void addrconf_forward_change(void)
 	}
 	read_unlock(&dev_base_lock);
 }
-
+#endif
 
 /* Nobody refers to this ifaddr, destroy it */