tcp_output: fix sparse warning for tcp_wfree

Fix sparse warning:
	* symbol 'tcp_wfree' was not declared. Should it be static?
Signed-off-by: Silviu-Mihai Popescu <silviupopescu1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -940,7 +940,7 @@ void __init tcp_tasklet_init(void)
  * We cant xmit new skbs from this context, as we might already
  * hold qdisc lock.
  */
-void tcp_wfree(struct sk_buff *skb)
+static void tcp_wfree(struct sk_buff *skb)
 {
 	struct sock *sk = skb->sk;
 	struct tcp_sock *tp = tcp_sk(sk);