QmlProjectManager: Fix the build error coming from -Werror

Change-Id: I46601bcbb9ce358d34211fdea76707abcb5ca709
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmlprojectmanager/buildsystem/qmlbuildsystem.cpp

@@ -141,6 +141,7 @@ void QmlBuildSystem::refresh(RefreshOptions options)
         break;
     case RefreshOptions::Project:
         initProjectItem();
+        [[fallthrough]];
     case RefreshOptions::Files:
         parseProjectFiles();
     }