ArduSub: adjust for when LTM telemetry is disabled

ArduSub/GCS_Sub.cpp

@@ -96,8 +96,10 @@ void GCS_Sub::update_vehicle_sensor_status_flags()
 #endif
 }
 
+#if AP_LTM_TELEM_ENABLED
 // avoid building/linking LTM:
 void AP_LTM_Telem::init() {};
+#endif
 #if AP_DEVO_TELEM_ENABLED
 // avoid building/linking Devo:
 void AP_DEVO_Telem::init() {};