Add missing lock in QFactoryLoader

libraryList is modified in a code section that was not yet properly locked.

This was overlooked by 7ca66b1e66.

Task-number: QTBUG-42855
Change-Id: I028ad21a7df314133931f860ad49453bb2db9da0
Pick-to: 5.15
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/plugin/qfactoryloader.cpp

@@ -296,6 +296,7 @@ void QFactoryLoader::update()
             }
             if (keyUsageCount || keys.isEmpty()) {
                 library->setLoadHints(QLibrary::PreventUnloadHint); // once loaded, don't unload
+                QMutexLocker locker(&d->mutex);
                 d->libraryList += library;
             } else {
                 library->release();