[BUGFIX] Regression in DataHandler

The fix for issue #53862 calls GeneralUtility::idnaEncode
without fully qualified class namespace.

Follow-up to: c99a07a9

Resolves: #55475
Releases: 6.1, 6.0
Change-Id: I8ba161ee73e7456da53d2182b4a22d87dad9d53c
Reviewed-on: https://review.typo3.org/27174
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters