[TCP]: syn_flood_warning is only needed if CONFIG_SYN_COOKIES is selected

  CC      net/ipv4/tcp_ipv4.o
  /pub/scm/linux/kernel/git/acme/net-2.6/net/ipv4/tcp_ipv4.c:665: warning:
  'syn_flood_warning' defined but not used

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>