Add default label to QtMsgType switch statement

This enables us to add a new QtInfoMsg message type to qtbase
without breaking qtxmlpatterns.

Change-Id: Ia4dcee9390eb6b7fc02cf10067317879e931a092
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>