Fix compilation USE_USB_SERIAL

Compiling with USE_USB_SERIAL for usb console on s2, would fail as init_usb_serial() is defined after it's called.

Safe for 0.6.5 inclusion.

Signed-off-by: Peter M <petermm@gmail.com>

Author: petermm

src/platforms/esp32/main/main.c

@@ -41,6 +41,7 @@
 // idf.py add-dependency esp_tinyusb
 // and enable CDC in menu config
 #ifdef USE_USB_SERIAL
+void init_usb_serial(void);
 #include "tinyusb.h"
 #include "tusb_cdc_acm.h"
 #include "tusb_console.h"