Ensure Arduino.h and configs are included for serial.cpp

Marlin/serial.h

@@ -23,6 +23,8 @@
 #ifndef __SERIAL_H__
 #define __SERIAL_H__
 
+#include "MarlinConfig.h"
+
 #ifdef USBCON
   #include "HardwareSerial.h"
   #if ENABLED(BLUETOOTH)