netfilter: ipset: fix linking with CONFIG_IPV6=n

Add a dummy ip_set_get_ip6_port function that unconditionally
returns false for CONFIG_IPV6=n and convert the real function
to ipv6_skip_exthdr() to avoid pulling in the ip6_tables module
when loading ipset.

Signed-off-by: Patrick McHardy <kaber@trash.net>