netfilter: nf_conntrack: fix compilation if sysctl are disabled

In (f94161c1

 netfilter: nf_conntrack: move initialization out of pernet
operations), some ifdefs were missing for sysctl dependent code.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>