Fix the X11TimestampUpdateTest

Summary:
It also searches for the platform plugin, so we need to ensure it finds
it in the build directory.

This was a regression caused by building all platform plugins in the
correct location.

Test Plan: strace on the failing test, verified correct plugin is loaded

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17573