SerialUSB: fix missing overload after zephyrproject-rtos#123

This would have been spotted if we extend the CI to compile all the exaples (TestClient in particular).

Author: facchinm

cores/arduino/SerialUSB.h

@@ -19,6 +19,7 @@ class SerialUSB_ : public ZephyrSerial {
 
 	operator bool() override;
 	size_t write(const uint8_t *buffer, size_t size) override;
+	size_t write(const uint8_t data) override { return write(&data, 1); }
 	void flush() override;
 
 protected: