[BUGFIX] Fix path resolving for Default.css in EXT:rtehtmlarea

Path calculation is missing the resolving of EXT: syntax.
Fix this by adding GeneralUtility::getFileAbsFileName.

Resolves: #78501
Releases: master
Change-Id: Ib377bdd13e002988c35515ad571df2b4a2262e17
Reviewed-on: https://review.typo3.org/50440


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>