Proper silencing for the switch-case fall-through warning

Summary:
This changeset silences the gcc warning for the case fall-through.
No idea if this will work without warning for msvc, but that doesn't change much compared to the previous state.
C++17 can use "fallthrough;" to specify an execution path that intentionally falls through the following case.
Q_FALLTHROUGH was introduced in QT5.8, thus can't be used for Raspbian, still using QT5.4.

Test Plan:
Without this change, gcc issues a warning on the case fall-through in function unParkMount.
With this change fall-through is acknlowledged and the warning doesn't appear.

Reviewers: mutlaqja, wreissenberger

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D15137