Add missing require for a new validator.
Add missing require for a new validator.

This prevents /manuals from 500'ing.

alphagov/specialist-publisher-rebuild,ministryofjustice/specialist-publisher,alphagov/specialist-publisher-rebuild,alphagov/specialist-publisher-rebuild,ministryofjustice/specialist-publisher,alphagov/specialist-publisher,ministryofjustice/specialist-publisher,alphagov/specialist-publisher,alphagov/manuals-publisher,ministryofjustice/specialist-publisher,alphagov/specialist-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/specialist-publisher-rebuild