Gerbview: fix a minor issue.

When the option "always hide all layers but active" is set, the active layer
changed using hotkeys or Toolbar layer selector did not update the visible
selection.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/18059

gerbview/gerbview_frame.cpp

@@ -877,7 +877,10 @@ void GERBVIEW_FRAME::SetActiveLayer( int aLayer, bool doLayerWidgetUpdate )
         UpdateXORLayers();
 
     if( doLayerWidgetUpdate )
+    {
         m_LayersManager->SelectLayer( aLayer );
+        m_LayersManager->OnLayerSelected();
+    }
 
     UpdateTitleAndInfo();