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.