can: slcan: remove useless header inclusions

Include only the necessary headers.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Link: https://lore.kernel.org/all/20220728070254.267974-3-dario.binacchi@amarulasolutions.com


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>