tty: add missing tty_driver include to tty_port.h

tty: add missing tty_driver include to tty_port.h

Include the definition of struct tty_driver in tty_port.h to keep the
header self-contained and avoid build breakage in case anyone includes
it before tty_driver.h.

Fixes: eb3b0d92c9c3 ("tty: tty_port: add workqueue to flip TTY buffer")
Cc: Xin Zhao <jackzxcui1989@163.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://patch.msgid.link/20260506124323.186703-1-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>