Fix inconsistent function signature in HardwareSerial.h

This causes an override mismatch for UARTClass::begin(const uint32_t) in UARTClass.h when compiling with Clang

Author: weliveindetail

Diff for: cores/arduino/HardwareSerial.h

@@ -26,7 +26,7 @@
 class HardwareSerial : public Stream
 {
   public:
-    virtual void begin(unsigned long);
+    virtual void begin(uint32_t);
     virtual void end();
     virtual int available(void) = 0;
     virtual int peek(void) = 0;