netfilter: ip6t_MASQUERADE: add dependency on conntrack module

After commit 4d3a57f2 ("netfilter: conntrack: do not enable connection
tracking unless needed") conntrack is disabled by default unless some
module explicitly declares dependency in particular network namespace.

Fixes: a357b3f8 ("netfilter: nat: add dependencies on conntrack module")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>