tcp: do not export sysctl_tcp_low_latency

Since commit b2fb4f54

 ("tcp: uninline tcp_prequeue()") we no longer
access sysctl_tcp_low_latency from a module.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>