Plane: add missing override keywords in afs

ArduPlane/afs_plane.h

@@ -30,14 +30,14 @@ public:
     AP_AdvancedFailsafe_Plane(AP_Mission &_mission, const AP_GPS &_gps);
 
     // called to set all outputs to termination state
-    void terminate_vehicle(void);
+    void terminate_vehicle(void) override;
     
 protected:
     // setup failsafe values for if FMU firmware stops running
-    void setup_IO_failsafe(void);
+    void setup_IO_failsafe(void) override;
 
     // return the AFS mapped control mode
-    enum control_mode afs_mode(void);
+    enum control_mode afs_mode(void) override;
 };
 
 #endif // ADVANCED_FAILSAFE