not build QQmlSettings when QT_NO_SETTINGS is defined

The current implementation is based on QSettings.

Change-Id: I2e45070dabaf915b1c1b9f4e7c63b6c3e36e48a0
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>