netfilter: conntrack: define variables exp_nat_nla_policy and any_addr with CONFIG_NF_NAT

gcc with W=1 and ! CONFIG_NF_NAT
net/netfilter/nf_conntrack_netlink.c:3463:32: error:
  ‘exp_nat_nla_policy’ defined but not used [-Werror=unused-const-variable=]
 3463 | static const struct nla_policy exp_nat_nla_policy[CTA_EXPECT_NAT_MAX+1] = {
      |                                ^~~~~~~~~~~~~~~~~~
net/netfilter/nf_conntrack_netlink.c:2979:33: error:
  ‘any_addr’ defined but not used [-Werror=unused-const-variable=]
 2979 | static const union nf_inet_addr any_addr;
      |                                 ^~~~~~~~

These variables use is controlled by CONFIG_NF_NAT, so should their definitions.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Florian Westphal <fw@strlen.de>