Adding safeguard for USBSerial

ref: espressif/arduino-esp32#8276 (comment)

Author: tobozo

src/M5CoreS3.h

@@ -1,6 +1,14 @@
 #ifndef _M5CORES3_H_
 #define _M5CORES3_H_
 
+#if ARDUINO_USB_CDC_ON_BOOT
+  #define USBSerial Serial
+#else
+  #include <HWCDC.h>
+  // the warning produced by using the 'inline' keyword is a necessary evil as (HWCDC)USBSerial is declared extern and can't be casted as static
+  inline HWCDC USBSerial;
+#endif
+
 #include <Arduino.h>
 #include <Wire.h>
 #include "FS.h"
@@ -40,4 +48,4 @@ extern M5CoreS3 M5;
 #define imu IMU
 #define Imu IMU
 
-#endif
+#endif