Fix some warnings on mac

These are mostly missing 'overrides'

src/commands/AudacityCommand.h

@@ -70,9 +70,9 @@ class AUDACITY_DLL_API AudacityCommand /* not final */ : public wxEvtHandler,
    //  virtual wxString GetFamily();
 
    //These two must be implemented by instances.
-   virtual wxString GetSymbol()
+   virtual wxString GetSymbol() override
    { wxFAIL_MSG( "Implement a Symbol for this command");return "FAIL";};
-   virtual wxString GetDescription()
+   virtual wxString GetDescription() override
    {wxFAIL_MSG( "Implement a Description for this command");return "FAIL";};
 
    // Name of page in the Audacity alpha manual

src/commands/Command.h

@@ -105,7 +105,7 @@ public:
    virtual ~CommandImplementation();
 
    /// An instance method for getting the command name (for consistency)
-   wxString GetName();
+   wxString GetName() override;
 
    /// Get the signature of the command
    CommandSignature &GetSignature() override;

src/commands/Validators.h

@@ -153,7 +153,7 @@ public:
 class BoolArrayValidator final : public Validator
 {
 public:
-   virtual bool Validate(const wxVariant &v)
+   virtual bool Validate(const wxVariant &v) override
    {
       wxString val;         // Validate a string of chars containing only 0, 1 and x.
       if (!v.Convert(&val))