can: add explicit copyrights to can headers

These files are copied to the source code of user space applications (in
this case can-utils) and so it makes sense to mention explicitly their
copyright. I added the terms of C code that was introduced in the same
commit as these headers.

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