Copy plastik window decoration to build directory

Summary:
This ensures that DontCrashAuroraeDestroyDecoTest can load the plastik
window decoration when run from build directory as it happens on
build.kde.org.

Test Plan: Test passes now when manipulating XDG_DATA_DIRS to hide system install

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16745