[BUGFIX] makeCategorizable: use $_EXTKEY for the extension-key

Variable $extensionKey currently used is not available there.

Change-Id: I172c674f8fca06e443ad797c96f54169a5ef82c9
Resolves: #58350
Reviewed-on: https://review.typo3.org/29737
Reviewed-by: Oliver Klee
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind