GN: Remove build/ dependency in "Null" GN file. This was breaking the Skia roller. Bug: angleproject:3943 Change-Id: I5b54feb2e4abc8a6bf157e35f69aa12c7334dda2 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2411474 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>