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