[thumbnail] Don't use QDir::separator() to compare paths

The plugin name always has forward slashes, even on Windows

Using the native separator causes the comparison to fail

thumbnail/thumbnail.cpp

@@ -654,7 +654,7 @@ ThumbCreatorWithMetadata* ThumbnailProtocol::getThumbCreator(const QString& plug
     ThumbCreatorWithMetadata *thumbCreator = nullptr;
     if (creator) {
         KPluginMetaData data;
-        if (plugin.contains(QLatin1String("kf5") + QDir::separator() + QLatin1String("thumbcreator"))) {
+        if (plugin.contains(QLatin1String("kf5/thumbcreator"))) {
             data = KPluginMetaData(plugin);
         } else {
             const QVector<KPluginMetaData> plugins = KIO::PreviewJob::availableThumbnailerPlugins();