ip: remove unused function prototype

ip_do_nat() function was removed prior to kernel 3.4. Remove the
unnecessary function prototype as well.
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -110,7 +110,6 @@ int ip_output(struct sock *sk, struct sk_buff *skb);
 int ip_mc_output(struct sock *sk, struct sk_buff *skb);
 int ip_do_fragment(struct sock *sk, struct sk_buff *skb,
 		   int (*output)(struct sock *, struct sk_buff *));
-int ip_do_nat(struct sk_buff *skb);
 void ip_send_check(struct iphdr *ip);
 int __ip_local_out(struct sk_buff *skb);
 int ip_local_out_sk(struct sock *sk, struct sk_buff *skb);