netfilter: connlimit: fix UP build

cannot use ARRAY_SIZE() if spinlock_t is empty struct.

Fixes: 1442e750 ("netfilter: connlimit: use keyed locks")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>