Add missing header to build config. Bug: angleproject:6185 Change-Id: Ia3292935489e808bc53a6df3723d81355789b460 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3036588 Commit-Queue: Jeff Gilbert <jgilbert@mozilla.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>