AP_Compass: add override keyword where required

libraries/AP_Compass/AP_Compass_AK8963.h

@@ -122,8 +122,8 @@ public:
     
     AP_HAL::Semaphore  *get_semaphore() override;
 
-    bool configure();
-    bool start_measurements();
+    bool configure() override;
+    bool start_measurements() override;
 
     // set device type within a device class
     void set_device_type(uint8_t devtype) override;

libraries/AP_Compass/AP_Compass_HIL.h

@@ -8,7 +8,7 @@ class AP_Compass_HIL : public AP_Compass_Backend
 {
 public:
     AP_Compass_HIL();
-    void read(void);
+    void read(void) override;
     bool init(void);
 
     // detect the sensor