[KickerDash] Add missed argument in addFavorite

Test Plan:
Before:
- Drag&Drop an item from the middle column into the Favorite column at the position 2 (index =1) -> it is always added at the last position

After:
- Drag&Drop an item from the middle column into the Favorite column at the position 2 (index =1) -> it is added at the wanted position 2

Reviewers: ngraham, hein, #plasma

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19893