Fix stupid copy paste bug in XDGShellV6 Client

Summary:
Wasn't spotted because of the bug in the test which is pretty much the
sole use of this class.

Test Plan: Fixed test now passes

Reviewers: KWin, romangg

Reviewed By: KWin, romangg

Subscribers: romangg, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13494