[BUGFIX] Incompatible declaration of PreviewController::initializeView

The declaration of PreviewController::initializeView is changed to be
functional with PHP 7.

Change-Id: I4b59600aa3d98dec6440551f4228c9aad6fd6714
Resolves: #73866
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/46990


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>