AP_HAL_Empty: add HAL_UART_STATS_ENABLED to disable stats gathering

libraries/AP_HAL_Empty/UARTDriver.cpp

@@ -30,7 +30,9 @@ size_t Empty::UARTDriver::write(const uint8_t *buffer, size_t size)
     return n;
 }
 
+#if HAL_UART_STATS_ENABLED
 void Empty::UARTDriver::uart_info(ExpandingString &str)
 {
     str.printf("EMPTY\n");
 }
+#endif

libraries/AP_HAL_Empty/UARTDriver.h

@@ -24,6 +24,8 @@ public:
     size_t write(uint8_t c) override;
     size_t write(const uint8_t *buffer, size_t size) override;
 
+#if HAL_UART_STATS_ENABLED
     // request information on uart I/O for one uart
     void uart_info(ExpandingString &str) override;
+#endif
 };