Staging: make usbip depend on CONFIG_NET

Thanks to Randy Dunlap for finding this problem.
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/usbip/Kconfig

 config USB_IP_COMMON
 	tristate "USB IP support (EXPERIMENTAL)"
-	depends on USB && EXPERIMENTAL
+	depends on USB && NET && EXPERIMENTAL
 	default N
 	---help---
 	  This enables pushing USB packets over IP to allow remote