[BUGFIX] Broken icons in extension manager

This fixes broken button group in chrome.
This is a follow up for http://review.typo3.org/34779

Resolves: #63430
Releases: master
Change-Id: Id7f98527c128f5513bc78429c16741a27646c644
Reviewed-on: http://review.typo3.org/34795
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>