UART: use arduino namespace

Author: facchinm

Diff for: cores/arduino/UART.h

@@ -27,6 +27,8 @@
 #include "api/HardwareSerial.h"
 #include "pins_arduino.h"
 
+using namespace arduino;
+
 // Define constants and variables for buffering incoming serial data.  We're
 // using a ring buffer (I think), in which head is the index of the location
 // to which to write the next incoming character and tail is the index of the