tcp/dccp: tw_timer_handler() is static

tw_timer_handler() is only used from net/ipv4/inet_timewait_sock.c

Fixes: 789f558c

 ("tcp/dccp: get rid of central timewait timer")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>