netfilter: xt_cluster: add dependency on conntrack module

The cluster match requires conntrack for matching packets. If the
netns does not have conntrack hooks registered, the match does not
work at all.

Implicitly load the conntrack hook for the family, exactly as many
other extensions do. This ensures that the match works even if the
hooks have not been registered by other means.

Signed-off-by: Martin Willi <martin@strongswan.org>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>