netfilter: conntrack: make nf_conntrack_id callable without a module dependency

While nf_conntrack_id() doesn't need any functionaliy from conntrack, it
does reside in nf_conntrack_core.c -- callers add a module
dependency on conntrack.

Followup patch will need to compute the conntrack id from nf_tables_trace.c
to include it in nf_trace messages emitted to userspace via netlink.

I don't want to introduce a module dependency between nf_tables and
conntrack for this.

Since trace is slowpath, the added indirection is ok.

One alternative is to move nf_conntrack_id to the netfilter/core.c,
but I don't see a compelling reason so far.

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