erp5_maileva_connector: allow hidden type `Link` on Maileva Connector
This poor type was likely forgotten on
https://lab.nexedi.com/nexedi/erp5/-/merge_requests/2044. Since both links are now
registered as child Link objects, updating the connector after upgrading led to
an error: Disallowed subobject type: Link on <Maileva SOAP Connector at XXX>.