ipvs: remove dependency on ip6_tables

This dependency was added because ipv6_find_hdr was in iptables specific
code but is no longer required

Fixes: f8f62675 ("ipv6: Move ipv6_find_hdr() out of Netfilter code.")
Fixes: 63dca2c0 ("ipvs: Fix faulty IPv6 extension header handling in IPVS")
Signed-off-by: Yaroslav Bolyukin <iam@lach.pw>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>