Instead of the Khronos gitlab repo.

The gitlab repo requires authentication, and seems to interact
badly with Google's extra SSH layers.

Change-Id: I982bf103d83ed33c7438562992c21f93abb05725
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38808Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>