Print a warning if runner is incompatible with KRunner
Useful for finding out why runner isn't loaded even though it's enabled in
settings. Can for example happen if a runner is installed by an application
that's compiled against a newer KRunner version than Plasma.
Test Plan: Try to load an incompatible runner plugin
Reviewed By: broulik
Differential Revision: https://phabricator.kde.org/D28624