usb gadget: g_cdc dependso on NET

g_cdc needs to depend on NET, otherwise net-related build errors happen:

ERROR: "netif_carrier_on" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "netif_carrier_off" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "netif_rx" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "alloc_etherdev_mq" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "free_netdev" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "register_netdev" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "print_mac" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "dev_kfree_skb_any" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "__alloc_skb" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "eth_type_trans" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "unregister_netdev" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "__netif_schedule" [drivers/usb/gadget/g_cdc.ko] undefined!
ERROR: "skb_put" [drivers/usb/gadget/g_cdc.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>