[BUGFIX] Ensure correct path to t3skin_override.css in BE

Change-Id: I01e9dec43356c8b3234b531b1c13519806dc22b8
Resolves: #61918
Reviewed-on: http://review.typo3.org/32994
Reviewed-by: Jo Hasenau <info@cybercraft.de>
Tested-by: Jo Hasenau <info@cybercraft.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>