netfilter: conntrack: timeout interface depend on CONFIG_NF_CONNTRACK_TIMEOUT

Now that cttimeout support for nft_ct is in place, these should depend
on CONFIG_NF_CONNTRACK_TIMEOUT otherwise we can crash when dumping the
policy if this option is not enabled.

[   71.600121] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
[...]
[   71.600141] CPU: 3 PID: 7612 Comm: nft Not tainted 4.18.0+ #246
[...]
[   71.600188] Call Trace:
[   71.600201]  ? nft_ct_timeout_obj_dump+0xc6/0xf0 [nft_ct]

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>