MAINTAINERS: CAN network layer: add missing header file can-ml.h

This patch add the can-ml.h to the list of maintained files of the CAN network
layer.

Fixes: ffd956ee

 ("can: introduce CAN midlayer private and allocate it automatically")
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://lore.kernel.org/r/20210111141930.693847-2-mkl@pengutronix.de


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