Fix cache directory calculation when adding to trash

Summary: The directory size was not added to the directory cache size since D24773

Test Plan:
Moved a folder to trash.
Its size is visible in
~/.local/share/Trash/directorysizes

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28973