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>

drivers/usb/gadget/Kconfig

@@ -588,6 +588,7 @@ config USB_G_PRINTER
 
 config USB_CDC_COMPOSITE
 	tristate "CDC Composite Device (Ethernet and ACM)"
+	depends on NET
 	help
 	  This driver provides two functions in one configuration:
 	  a CDC Ethernet (ECM) link, and a CDC ACM (serial port) link.