[NETFILTER]: Ipv6-related xt_hashlimit compilation fix.

The hashlimit_ipv6_mask() is called from under IP6_NF_IPTABLES config
option, but is not under it by itself.

gcc warns us about it :) :
net/netfilter/xt_hashlimit.c:473: warning: "hashlimit_ipv6_mask" defined but not used

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>