can: add explicit copyrights to can userspace header

This is in the spirit of commit 2485602f (can: add explicit
copyrights to can headers). It seems I have missed can.h back then.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>