dataengines/potd: Use references in `sendXmlRequest`

dataengines/potd/flickrprovider.cpp

@@ -39,7 +39,7 @@ FlickrProvider::FlickrProvider(QObject *parent, const QVariantList &args)
 
 FlickrProvider::~FlickrProvider() = default;
 
-void FlickrProvider::sendXmlRequest(QString apiKey, QString apiSecret)
+void FlickrProvider::sendXmlRequest(const QString &apiKey, const QString &apiSecret)
 {
     Q_UNUSED(apiSecret);
     if (apiKey.isNull()) {

dataengines/potd/flickrprovider.h

@@ -40,7 +40,7 @@ public:
     ~FlickrProvider() override;
 
 private:
-    void sendXmlRequest(QString apiKey, QString apiSecret);
+    void sendXmlRequest(const QString &apiKey, const QString &apiSecret);
     void xmlRequestFinished(KJob *job);
     void imageRequestFinished(KJob *job);