[BUGFIX] Get the correct class of the new "add new element" icon

Change-Id: I7a0aff014d83b9b2079fd890c18642c89aa3eb09
Resolves: #63593
Releases: master
Reviewed-on: http://review.typo3.org/35069
Reviewed-by: Jo Hasenau <info@cybercraft.de>
Tested-by: Jo Hasenau <info@cybercraft.de>