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>

MAINTAINERS

@@ -1703,6 +1703,7 @@ F:	include/linux/can.h
 F:	include/linux/can/core.h
 F:	include/linux/can/bcm.h
 F:	include/linux/can/raw.h
+F:	include/linux/can/gw.h
 
 CAN NETWORK DRIVERS
 M:	Wolfgang Grandegger <wg@grandegger.com>