selftests: netfilter: fix config fragment CONFIG_NF_TABLES_INET

In selftests the config fragment for netfilter was added as
NF_TABLES_INET=y and this patch correct it as CONFIG_NF_TABLES_INET=y

Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>