gvc-level-bar: define priv->scale as a GvcLevelScale

That's cleaner and fix a sign-compare warning in gcc.

https://bugzilla.gnome.org/show_bug.cgi?id=666240

panels/sound/gvc-level-bar.c

@@ -65,7 +65,7 @@ struct GvcLevelBarPrivate
         GtkOrientation orientation;
         GtkAdjustment *peak_adjustment;
         GtkAdjustment *rms_adjustment;
-        int            scale;
+        GvcLevelScale  scale;
         gdouble        peak_fraction;
         gdouble        rms_fraction;
         gdouble        max_peak;