com20020: Fix allyesconfig build failure.

Reported by Stephen Rothwell.

Due to missing 'extern' in the com20020_netdev_ops declaration,
each file that includes linux/com20020.h gets another copy
defined in it's resulting object file.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/com20020.h

@@ -29,7 +29,7 @@
 
 int com20020_check(struct net_device *dev);
 int com20020_found(struct net_device *dev, int shared);
-const struct net_device_ops com20020_netdev_ops;
+extern const struct net_device_ops com20020_netdev_ops;
 
 /* The number of low I/O ports used by the card. */
 #define ARCNET_TOTAL_SIZE 8