selftests: explicitly require kernel features needed by udpgro tests

commit 3327a9c4 ("selftests: add functionals test for UDP GRO")
make use of ipv6 NAT, but such a feature is not currently implied by
selftests. Since the 'ip[6]tables' commands may actually create nft rules,
depending on the specific user-space version, let's pull both NF and
NFT nat modules plus the needed deps.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: 3327a9c4 ("selftests: add functionals test for UDP GRO")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>