usb: serial: Include <linux/usb.h> in <linux/usb/serial.h>

usb: serial: Include <linux/usb.h> in <linux/usb/serial.h>

All consumers of the latter also include the former, but without that
struct usb_driver and struct usb_device_id (and maybe more) are not
defined. Add an include for <linux/usb.h> to make the header
self-contained.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://patch.msgid.link/82219ab65d16ee5bfe5a35d11bc938baac3fd3bc.1782808461.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König (The Capable Hub) <u.kleine-koenig@baylibre.com>