can: fix sparse warning for cgw_list

Make cgw_list static to remove the following sparse warning:
net/can/gw.c:69:1: warning: symbol 'cgw_list' was not declared.
Should it be static?

Signed-off-by: Daniel Baluta <dbaluta@ixiacom.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>