Fix mac build breakage of 3b312f9d1b ...

... Warnings have been treated as errors in Xcode project settings since 2.3.1,
and there ware warnings about using override on only some but not all overridden
virtual functions in the PrefsPanel subclasses.

src/prefs/DevicePrefs.h

@@ -27,8 +27,8 @@ class DevicePrefs final : public PrefsPanel
  public:
    DevicePrefs(wxWindow * parent, wxWindowID winid);
    virtual ~DevicePrefs();
-   virtual ComponentInterfaceSymbol GetSymbol();
-   virtual wxString GetDescription();
+   ComponentInterfaceSymbol GetSymbol() override;
+   wxString GetDescription() override;
 
    bool Commit() override;
    wxString HelpPageName() override;