Make Prison a required dependency

7bd9bea0 started using Prison
unconditionally via QML import. So if Prison isn't installed the
clipboard applet stops working, complaining that "org.kde.prison is
not installed".
While it would certainly be possible to add another call to
"find_package(KF5Prison)" with TYPE RUNTIME, the result in
feature_summary would be very confusing, because OPTIONAL has a higher
priority and the runtime hint would vanish if you build with e.g.
-DCMAKE_DISABLE_FIND_PACKAGE_KF5Prison=TRUE.

(cherry picked from commit 63f1b860)