bb_retimer: add forward decl for usb_mux
This adds a forward declaration for struct usb_mux used in a function
prototype.
BRANCH=none
TEST=buildall passes
#ifndef __CROS_EC_DRIVER_RETIMER_BB_RETIMER_PUBLIC_H
#define __CROS_EC_DRIVER_RETIMER_BB_RETIMER_PUBLIC_H
+struct usb_mux;
+
/* Supported USB retimer drivers */
extern const struct usb_mux_driver bb_usb_retimer;
