exp/shiny: correct cast for vendored xproto

Change-Id: Iaabc9136debfffc64ccb8b93f2a303baa6de23c0
Reviewed-on: https://go-review.googlesource.com/16552
Reviewed-by: Sebastien Binet <seb.binet@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
1 file changed