[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: David S. Miller <davem@davemloft.net>