Do not embed the Desktop style into resources on irrelevant platforms

The Desktop style is not built on these touch/mobile platforms, or when
QtWidgets is not enabled, so it's pointless to embed these QML files.

Change-Id: Iac11473b3a9791745a107e0f7ac1cd34a91a9a5d
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>