[TASK] Use self:: where possible in GeneralUtility

It's common sense to use self:: where possible because it's quicker and
makes code better understandable.

This change fixes 2 violations in \TYPO3\CMS\Core\Utility\GeneralUtility

Releases: 6.2
Resolves: #59001
Change-Id: Ie56a6697186426e3ff082b1694572c885c8420f5
Reviewed-on: https://review.typo3.org/30257
Reviewed-by: Christian Kuhn
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters