[NET]: Fix allnoconfig build on powerpc and avr32

As reported by Haavard Skinnemoen and Stephen Rothwell:

> allnoconfig fails with
>
> include/linux/netdevice.h:843: error: implicit declaration of function 'dev_net'
>
> which seems to be because the definition of dev_net is inside #ifdef
> CONFIG_NET, while next_net_device, which calls it, is not.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -827,6 +827,7 @@ struct packet_type {
 extern rwlock_t				dev_base_lock;		/* Device list lock */
 
 
+#ifdef CONFIG_NET
 #define for_each_netdev(net, d)		\
 		list_for_each_entry(d, &(net)->dev_base_head, dev_list)
 #define for_each_netdev_safe(net, d, n)	\
@@ -850,6 +851,7 @@ static inline struct net_device *first_net_device(struct net *net)
 	return list_empty(&net->dev_base_head) ? NULL :
 		net_device_entry(net->dev_base_head.next);
 }
+#endif
 
 extern int 			netdev_boot_setup_check(struct net_device *dev);
 extern unsigned long		netdev_boot_base(const char *prefix, int unit);