Fix scrollbars not appearing initially in PANEL_SETUP_LAYERS.

(cherry picked from commit a4e2d54221)

Co-authored-by: Alex Shvartzkop <dudesuchamazing@gmail.com>

pcbnew/dialogs/panel_setup_layers.cpp

@@ -463,6 +463,8 @@ void PANEL_SETUP_LAYERS::initialize_layers_controls()
     }
 
     Thaw();
+
+    m_LayersSizer->FitInside( m_LayersListPanel );
     m_LayersListPanel->Layout();
 }