AP_HAL_Linux: AnalogIn_ADS1115: return 5V on board_voltage

board voltage is not available - don't return 0V since that prevents
arming.

libraries/AP_HAL_Linux/AnalogIn_ADS1115.h

@@ -25,14 +25,13 @@ private:
 class AnalogIn_ADS1115: public AP_HAL::AnalogIn {
 public:
     AnalogIn_ADS1115();
-    void init();
-    AP_HAL::AnalogSource* channel(int16_t n);
+
+    void init() override;
+    AP_HAL::AnalogSource *channel(int16_t n) override;
 
     /* Board voltage is not available */
-    float board_voltage(void)
-    {
-        return 0.0f;
-    }
+    float board_voltage() override { return 5.0f; }
+
 private:
     uint8_t _channels_number;
     void _update();