mesa: Require current libxcb.

mesa: Require current libxcb.

Without that, people with buggy apps that looked at just the server
string for GLX_ARB_create_context would call this function that just
threw an error when you tried to make a context.  Google shows plenty
of complaints about this.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>