Problem with xorg backport

[Anthony DeRobertis <aderobertis@metrics.net>]

The xorg backport lets you install libx11-dev along with the old x-dev 
4.3.0. The problem is that libx11-dev expects there to be a  _X_SENTINEL 
macro defined in Xfuncproto.h, whereas the old x-dev doesn't have one.

Please add a versioned depends on x-dev to libx11-dev.