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