GN: Remove build/ dependency in "Null" GN file.
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 <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle