Fixed bin/otrs.PostmasterMailbox.pl script.

After the port to the ObjectManager, this script is no longer working.
It generates the error message

```
Can't locate object method "new" via package "Kernel::System::MailAccount" (perhaps you
forgot to load "Kernel::System::MailAccount"?) at
/home/michiel/otrs/bin/otrs.PostMasterMailbox.pl line 109.
```

So I'm loading it now.