[kcm/standardactions] Don't register kcminit hook when we don't have one

Summary:
There is no exported function called standard_actions. kcminit will link
against this library, fail to find a method to run and do nothing.

Test Plan: Grepped for method

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27007