ps8xxx: add forward declare 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_TCPM_PS8XXX_PUBLIC_H
#define __CROS_EC_DRIVER_TCPM_PS8XXX_PUBLIC_H
+struct usb_mux;
+
/* I2C interface */
#define PS8751_I2C_ADDR1_P1_FLAGS 0x09
#define PS8751_I2C_ADDR1_P2_FLAGS 0x0A
