Add UART_3 define for compilation fix

Author: adustm

hal/targets/hal/TARGET_STM/TARGET_STM32F3/TARGET_NUCLEO_F303K8/PeripheralNames.h

@@ -48,7 +48,7 @@ typedef enum {
 
 typedef enum {
     UART_1 = (int)USART1_BASE,
-    UART_2 = (int)USART2_BASE
+    UART_2 = (int)USART2_BASE,
 } UARTName;
 
 #define STDIO_UART_TX  PA_2