selftests: netfilter: Enable CONFIG_INET_SCTP_DIAG

The config snippet specifies CONFIG_SCTP_DIAG. This was never an option.

Replace CONFIG_SCTP_DIAG with the intended CONFIG_INET_SCTP_DIAG.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>