netfilter: fix compilation error with NAT=n

This patch fixes the compilation of ctnetlink when the NAT support
is not enabled.

/home/benh/kernels/linux-powerpc/net/netfilter/nf_conntrack_netlink.c:819: warning: enum nf_nat_manip_type\u2019 declared inside parameter list
/home/benh/kernels/linux-powerpc/net/netfilter/nf_conntrack_netlink.c:819: warning: its scope is only this definition or declaration, which is probably not what you want

Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reported by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>