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: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>