MAINTAINERS: Add can-gw include to maintained files

Commit c1aabdf3 (can-gw: add netlink based
CAN routing) added a new include file that's neither referenced by any of
the CAN maintainers.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>