HAL_SITL: implement valid_pin()

libraries/AP_HAL_SITL/GPIO.h

@@ -18,6 +18,8 @@ public:
     /* return true if USB cable is connected */
     bool usb_connected(void) override;
 
+    bool valid_pin(uint8_t pin) const override { return pin < 16; }
+    
 private:
     SITL_State *_sitlState;
 
@@ -35,4 +37,4 @@ public:
 private:
     uint8_t _pin;
 };
-#endif
+#endif