The BUILD.gn files used to ignore the platform entirely. All that was
necessary was to point them at the proper include directory for Mac.

Bug chromium:966543

Change-Id: I755a432cc7d01ce094d7191fd48d0702d8a1ae3e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31889
Presubmit-Ready: Sean Risser <srisser@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Sean Risser <srisser@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>