[BUGFIX] Show icons of backend layouts

Since changing the implementation of backend layouts (#37208) the icons
of backend layouts are not shown anymore.

This change fixed the path so that it is working again.

Change-Id: I7de03ac7fa90c5f62ec66b14dd391a316b858a98
Resolves: #52953
Releases: 6.2
Reviewed-on: https://review.typo3.org/25087
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters