Also make sure that EGL_EGLEXT_PROTOTYPES is always defined so that
extension functions can be used without loading them.

BUG=angleproject:970

Change-Id: I33fa3e8ed68aeda55ad69e1bc7826646f5e3ce29
Reviewed-on: https://chromium-review.googlesource.com/287161Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>