types.db: Reintroduced the "spam_check" type.

types.db: Reintroduced the "spam_check" type.

This is used by the "email" plugin. Apparently, it got lost when converting
the plugin to the v4 interface: Instead of "spam_check", "email_check" had
been added to types.db. I did not remove "email_check" though as this would
introduce a regression - though very unlikely, somebody out there might be
using that type.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>