fixed InertialSensor stub build

ArduCopter/ArduCopter.pde

@@ -217,8 +217,8 @@ AP_TimerProcess timer_scheduler;
 	AP_GPS_HIL              g_gps_driver(NULL);
 	AP_Compass_HIL          compass; // never used
 	AP_IMU_Shim             imu; // never used
-  AP_InertialSensorStub ins;
+    AP_InertialSensor_Stub ins;
-  AP_PeriodicProcessStub timer_scheduler;
+    AP_PeriodicProcessStub timer_scheduler;
 	#ifdef OPTFLOW_ENABLED
 		AP_OpticalFlow_ADNS3080 optflow;
 	#endif

libraries/AP_InertialSensor/AP_InertialSensor_Stub.cpp

@@ -5,7 +5,7 @@ void AP_InertialSensor_Stub::init( AP_PeriodicProcess * scheduler ) {}
 
 /*================ AP_INERTIALSENSOR PUBLIC INTERFACE ==================== */
 
-bool AP_InertialSensor_Stub::update( void ) {}
+bool AP_InertialSensor_Stub::update( void ) { return true; }
 
 
 float AP_InertialSensor_Stub::gx() { return 0.0f; }