AP_Notify: add override keyword where required

libraries/AP_Notify/AP_BoardLED.h

@@ -26,10 +26,10 @@ class AP_BoardLED: public NotifyDevice
 {
 public:
     // initialise the LED driver
-    bool init(void);
+    bool init(void) override;
 
     // should be called at 50Hz
-    void update(void);
+    void update(void) override;
 
 private:
     // counter incremented at 50Hz

libraries/AP_Notify/AP_BoardLED2.h

@@ -28,10 +28,10 @@ class AP_BoardLED2: public NotifyDevice
 {
 public:
     // initialise the LED driver
-    bool init(void);
+    bool init(void) override;
 
     // should be called at 50Hz
-    void update(void);
+    void update(void) override;
 
 private:
     // counter incremented at 50Hz

libraries/AP_Notify/Buzzer.h

@@ -30,10 +30,10 @@ public:
     Buzzer() {}
 
     /// init - initialise the buzzer
-    bool init(void);
+    bool init(void) override;
 
     /// update - updates buzzer according to timed_updated.  Should be called at 50Hz
-    void update();
+    void update() override;
 
     /// on - turns the buzzer on or off
     void on(bool on_off);

libraries/AP_Notify/Display.h

@@ -13,8 +13,8 @@ class Display: public NotifyDevice {
 public:
     friend class Display_Backend;
 
-    bool init(void);
+    bool init(void) override;
-    void update();
+    void update() override;
 
 private:
     void draw_char(uint16_t x, uint16_t y, const char c);

libraries/AP_Notify/ExternalLED.h

@@ -31,10 +31,10 @@ public:
     ExternalLED() : _pattern(NONE) {}
 
     // initialise the LED driver
-    bool init(void);
+    bool init(void) override;
 
     // should be called at 50Hz
-    void update(void);
+    void update(void) override;
 
 private:
 

libraries/AP_Notify/RGBLed.h

@@ -27,14 +27,14 @@ public:
     RGBLed(uint8_t led_off, uint8_t led_bright, uint8_t led_medium, uint8_t led_dim);
 
     // init - initialised the LED
-    virtual bool init(void);
+    virtual bool init(void) override;
 
     // set_rgb - set color as a combination of red, green and blue levels from 0 ~ 15
     virtual void set_rgb(uint8_t red, uint8_t green, uint8_t blue);
 
     // update - updates led according to timed_updated.  Should be
     // called at 50Hz
-    virtual void update();
+    virtual void update() override;
 
     // handle LED control, only used when LED_OVERRIDE=1
     virtual void handle_led_control(mavlink_message_t *msg) override;

libraries/AP_Notify/ToneAlarm.h

@@ -30,10 +30,10 @@ public:
     bool init(void) override;
 
     /// update - updates led according to timed_updated.  Should be called at 50Hz
-    void update();
+    void update() override;
 
     // handle a PLAY_TUNE message
-    void handle_play_tune(mavlink_message_t *msg);
+    void handle_play_tune(mavlink_message_t *msg) override;
 
 private:
     /// play_tune - play one of the pre-defined tunes