AP_HAL_VRBRAIN: add override keyword

libraries/AP_HAL_VRBRAIN/UARTDriver.h

@@ -38,7 +38,7 @@ public:
     }
 
     void set_flow_control(enum flow_control flow_control);
-    enum flow_control get_flow_control(void) { return _flow_control; }
+    enum flow_control get_flow_control(void) override { return _flow_control; }
 
     void configure_parity(uint8_t v);
     void set_stop_bits(int n);