[NET]: Make NETDEVICES depend on NET.

Enabling drivers from "Devices > Networking" (in menuconfig), for
example SLIP and/or PLIP, throws link time errors when CONFIG_NET itself
is =n. Have CONFIG_NETDEVICES depend on CONFIG_NET.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>