This makes it so it is the only file including glx.h.

BUG=angleproject:892

Change-Id: I5c45b659ec102b7a2c92e010a6843fa21b7c664b
Reviewed-on: https://chromium-review.googlesource.com/271418Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>