Dialogs: Use native rendering on non-mobile platforms

Follow the same rule as in QtQuick.Controls: change
332ef31f0d1645eded5588729176134bbdcd38a2

Task-number: QTBUG-41734
Change-Id: I6d80e0312d3986b90952e43ffbddec1fe007fddb
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>