netfilter: cttimeout: Make NF_CT_NETLINK_TIMEOUT depend on NF_CONNTRACK_TIMEOUT

With this, remove ifdef for CONFIG_NF_CONNTRACK_TIMEOUT in
nfnetlink_cttimeout. This is also required for moving ctnl_untimeout
from nfnetlink_cttimeout to nf_conntrack_timeout.

Signed-off-by: Harsha Sharma <harshasharmaiitr@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>