[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