staging: usbip: userspace: add hwdata as optional dependency in README

This is an optional dependency since USB/IP can fully work without
it. However, it is needed to display device information such as
vendor.

Signed-off-by: Valentina Manea <valentina.manea.m@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>