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>

net/can/gw.c

@@ -66,7 +66,7 @@ MODULE_LICENSE("Dual BSD/GPL");
 MODULE_AUTHOR("Oliver Hartkopp <oliver.hartkopp@volkswagen.de>");
 MODULE_ALIAS("can-gw");
 
-HLIST_HEAD(cgw_list);
+static HLIST_HEAD(cgw_list);
 static struct notifier_block notifier;
 
 static struct kmem_cache *cgw_cache __read_mostly;