Starting from [1], GN is able to ensure that the inclusion or headers
with relative paths is matched by a GN dependency.

This CL fixes the libyuv build graph according to GN.

[1] - https://chromium-review.googlesource.com/c/chromium/src/+/827014

Bug: webrtc:8850
Change-Id: I7e0f9a8bd1ae2d205213dc75bf322be587efd1f5
Reviewed-on: https://chromium-review.googlesource.com/903843Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>