The necessary submodules should be downloaded automatically.

Bug: b/141361933
Change-Id: Icd4690dccb851ecdc67a8f216a5727f1d8b7142a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43068
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>