add logins relation to service

Prevents creds command from throwing a stack trace complaining about lack of relation for Mdm::Service :logins and Metasploit::Credential::Login :service.

```
[-] Error while running command creds: Could not find the inverse association for service (:logins in Mdm::Service)
```

After manually pry-ing the class and inserting the AssociationReflection everything works as expected.