Fixed argument type hinting

The incorrect type hinting in `SyncToMailChimpFieldProvider::saveAdditionalFields()` led to a PHP Fatal error in two of our TYPO3 6.1.7 installations (invalid namespace).

But more importantly, **the whole Scheduler task setup seems to get broken by the t3chimp** extension (tested in two separate TYPO3 6.1.7 installations). I did not have the time to dig into the code any further though ... Can you confirm this?