Don't show User Feedback page if KUserFeedback isn't available
Some distros patch this out (e.g. openSUSE) so for them, we don't want to show the user a blank and broken-looking page. This commit handles that case appropriately by marking KUserFeedback as an optional dependency, and only embedding its KCM if it's found.
BUG: 462805 FIXED-IN: 5.27