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>