staging/usbip: add uapi header to export usbip kernel interfaces

usbip userspace has duplicated enum definition to report usbip device
status maintained by the kernel. Adding an usbip uapi header file will
define the kernel - userspace interface for this device status. This
new uapi file is added under usbip/uapi to keep the staging tree code
self-contained. When usbip moves to mainline drivers, this file should
be moved under uapi/linux

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>