netfilter: nf_conntrack: restore nf_conntrack_htable_size as exported symbol

This is required to iterate over the hash table in cttimeout, ctnetlink
and nf_conntrack_ipv4.

>> ERROR: "nf_conntrack_htable_size" [net/netfilter/nfnetlink_cttimeout.ko] undefined!
   ERROR: "nf_conntrack_htable_size" [net/netfilter/nf_conntrack_netlink.ko] undefined!
   ERROR: "nf_conntrack_htable_size" [net/ipv4/netfilter/nf_conntrack_ipv4.ko] undefined!

Fixes: adf05168

 ("netfilter: remove ip_conntrack* sysctl compat code")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>