Tracker: compilation fixes for ServoRelayEvents in superclass

AntennaTracker/GCS_Mavlink.h

@@ -18,6 +18,7 @@ protected:
 
     AP_Mission *get_mission() override { return nullptr; };
     AP_Rally *get_rally() const override { return nullptr; };
+    AP_ServoRelayEvents *get_servorelayevents() const override { return nullptr; }
 
     uint8_t sysid_my_gcs() const override;