Q_ATTRIBUTE_FORMAT_PRINTF is added to qdoc's list of ignored
directives. Without it, qdoc fails to parse a few function
declarations.

Change-Id: Ie24a58d9a5dcdffb8c103b2f5e34dd09b9471f06
Task-number: QTBUG-46920
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>