[QDeclarativeSound] Pass QString by const-ref

Change-Id: Ic600e62420c2e3ccc66f037e0fe44374e505c0d1
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>

src/imports/audioengine/qdeclarative_sound_p.h

@@ -112,7 +112,7 @@ public:
     void setName(const QString& name);
 
     QString attenuationModel() const;
-    void setAttenuationModel(QString attenuationModel);
+    void setAttenuationModel(const QString &attenuationModel);
 
     QDeclarativeSoundCone* cone() const;