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

MAINTAINERS

@@ -3960,6 +3960,7 @@ W:	https://github.com/linux-can
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/mkl/linux-can.git
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/mkl/linux-can-next.git
 F:	Documentation/networking/can.rst
+F:	include/linux/can/can-ml.h
 F:	include/linux/can/core.h
 F:	include/linux/can/skb.h
 F:	include/net/netns/can.h