cdc: Fix build warning.

In file included from drivers/usb/gadget/function/u_serial.h:16:0,
from drivers/usb/gadget/function/f_acm.c:23:
>> include/linux/usb/cdc.h:47:5: warning: 'struct usb_interface' declared inside parameter list
int buflen);
^
>> include/linux/usb/cdc.h:47:5: warning: its scope is only this definition or declaration, which is probably not what you want

Reported-by: kbuild test robot <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/usb/cdc.h

···
 	bool phonet_magic_present;
 };
 
-
+struct usb_interface;
 int cdc_parse_cdc_header(struct usb_cdc_parsed_header *hdr,
 				struct usb_interface *intf,
 				u8 *buffer,