[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