kcm baloo: Refuse to add root directory
Summary:
Before this change you could either have or not have the root folder in
the list of excluded folders and get the same behaviour - that was
confusing.
Test Plan: Manually tested by me.
Reviewers: plasma-devel, mart
Reviewed By: mart
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D8326