netfilter: nfnetlink_queue: fix compilation with NF_CONNTRACK disabled

In "9cb01766

 netfilter: add glue code to integrate nfnetlink_queue and ctnetlink"
the compilation with NF_CONNTRACK disabled is broken. This patch fixes this
issue.

I have moved the conntrack part into nfnetlink_queue_ct.c to avoid
peppering the entire nfnetlink_queue.c code with ifdefs.

I also needed to rename nfnetlink_queue.c to nfnetlink_queue_pkt.c
to update the net/netfilter/Makefile to support conditional compilation
of the conntrack integration.

This patch also adds CONFIG_NETFILTER_QUEUE_CT in case you want to explicitly
disable the integration between nf_conntrack and nfnetlink_queue.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>