netfilter: nf_conntrack_sip: remove direct dependency on IPv6

Previous implementation was not usable with CONFIG_IPV6=m.

Fixes: a3419ce3 ("netfilter: nf_conntrack_sip: add sip_external_media logic")
Signed-off-by: Alin Nastac <alin.nastac@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>