tcp: make function tcp_retransmit_stamp() static

Fixes the following sparse warnings:

net/ipv4/tcp_timer.c:25:5: warning:
 symbol 'tcp_retransmit_stamp' 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_timer.c

@@ -22,7 +22,7 @@
 #include <linux/gfp.h>
 #include <net/tcp.h>
 
-u32 tcp_retransmit_stamp(const struct sock *sk)
+static u32 tcp_retransmit_stamp(const struct sock *sk)
 {
 	u32 start_ts = tcp_sk(sk)->retrans_stamp;