net/sched: Make NET_ACT_CT depends on NF_NAT

If NF_NAT is m and NET_ACT_CT is y, build fails:

net/sched/act_ct.o: In function `tcf_ct_act':
act_ct.c:(.text+0x21ac): undefined reference to `nf_ct_nat_ext_add'
act_ct.c:(.text+0x229a): undefined reference to `nf_nat_icmp_reply_translation'
act_ct.c:(.text+0x233a): undefined reference to `nf_nat_setup_info'
act_ct.c:(.text+0x234a): undefined reference to `nf_nat_alloc_null_binding'
act_ct.c:(.text+0x237c): undefined reference to `nf_nat_packet'

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: b57dc7c1 ("net/sched: Introduce action ct")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>