[NETFILTER]: xt_hashlimit: fix ip6tables dependency

IP6_NF_IPTABLES=m, CONFIG_NETFILTER_XT_MATCH_HASHLIMIT=y results in a
linker error since ipv6_find_hdr is defined in ip6_tables.c. Fix similar
to Adrian Bunk's H.323 conntrack patch: selecting ip6_tables to be build
as module requires hashlimit to be built as module as well.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>