[NETFILTER]: xtables: fixes warning on compilation of hashlimit

To use ipv6_find_hdr(), IP6_NF_IPTABLES is necessary.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>