openvswitch: Replace removed NF_NAT_NEEDED with IS_ENABLED(CONFIG_NF_NAT)

Commit 4806e975 ("netfilter: replace NF_NAT_NEEDED with
IS_ENABLED(CONFIG_NF_NAT)") removed CONFIG_NF_NAT_NEEDED, but a new user
popped up afterwards.

Fixes: fec9c271

 ("openvswitch: load and reference the NAT helper.")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Florian Westphal <fw@strlen.de>
Acked-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>