GN follow up to the corresponding CMake change:
https://swiftshader-review.googlesource.com/c/SwiftShader/+/32568

Bug b/134929443

Change-Id: Ic1eabafaef8e2eda92199434359456c1ba10e875
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34110Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>