tcp: make local function tcp_recv_timestamp static

Fixes the following sparse warning:

net/ipv4/tcp.c:1736:6: warning:
 symbol 'tcp_recv_timestamp' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1733,8 +1733,8 @@ static void tcp_update_recv_tstamps(struct sk_buff *skb,
 }
 
 /* Similar to __sock_recv_timestamp, but does not require an skb */
-void tcp_recv_timestamp(struct msghdr *msg, const struct sock *sk,
-			struct scm_timestamping *tss)
+static void tcp_recv_timestamp(struct msghdr *msg, const struct sock *sk,
+			       struct scm_timestamping *tss)
 {
 	struct timeval tv;
 	bool has_timestamping = false;