Print a warning if runner is incompatible with KRunner

Summary:
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

Reviewers: broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28624