[BUGFIX] RTE: Quickedit mode - RTE has no user CSS

Fix wrong absolute path calculation.

Releases: master, 6.2
Resolves: #56408
Change-Id: Ia96197f0dcf81bb8b52b511c75ae303a85ec9b97
Reviewed-on: http://review.typo3.org/33568
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>