netfilter: replace NF_NAT_NEEDED with IS_ENABLED(CONFIG_NF_NAT)

NF_NAT_NEEDED is true whenever nat support for either ipv4 or ipv6 is
enabled.  Now that the af-specific nat configuration switches have been
removed, IS_ENABLED(CONFIG_NF_NAT) has the same effect.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>