[BUGFIX] Fixed warning in docHeaderButtonsHook

Resolves: #61160
Change-Id: Iccaeeeac7a002e5bf47433b16b5bf80f7b390260
Reviewed-on: http://review.typo3.org/32351
Reviewed-by: Armin Ruediger Vieweg <armin@v.ieweg.de>
Tested-by: Armin Ruediger Vieweg <armin@v.ieweg.de>