Don't mark MinGW packages as to be installed by default

The MinGW toolchain should be only installed if the according
Qt package is installed.

Change-Id: I33b7390967e754707ceb9a36a9c651e59c99e753
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>