[TASK] Use GeneralUtility namespace in Installer.php

This makes Installer.php better readable due to shorter lines.

Change-Id: Iff1816f5c5d4108b60cc639be57f5c534b7312e0
Resolves: #47845
Releases: 6.2
Reviewed-on: https://review.typo3.org/20438
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn