[effects/flipswitch] Add a const ref

Summary: Add a const reference to match prototype of KGlobalAccel::globalShortcutChanged.

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26668