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>

src/tools/qdoc/qdocindexfiles.cpp

@@ -980,6 +980,7 @@ bool QDocIndexFiles::generateIndexSection(QXmlStreamWriter& writer,
             if (!brief.isEmpty())
                 writer.writeAttribute("brief", brief);
         }
+        break;
     case Node::QmlModule:
         {
             const QmlModuleNode* qmn = static_cast<const QmlModuleNode*>(node);