qdoc: Missing break in qdoc switch statement

There was a missing break statement in the function that generates the
.index file, which caused qdoc to output extra attributes in the
<module> element.

Change-Id: I110c15c67a228249bfe0c7da138f2ca0b4921371
Task-number: QTBUG-42625
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>