HAL_SITL: flow control is enabled on SITL

faster parameter download

libraries/AP_HAL_SITL/UARTDriver.h

@@ -56,6 +56,8 @@ public:
     // file descriptor, exposed so SITL_State::loop_hook() can use it
     int _fd;
 
+    enum flow_control get_flow_control(void) { return FLOW_CONTROL_ENABLE; }
+    
 private:
     uint8_t _portNumber;
     bool _connected = false; // true if a client has connected