Disable nullability-completeness on win clang builds.

The latest third party VMA triggers this warning. It does not impact
Skia at all and we don't use the nullable specifiers. Thus it is safe
to just ignore this warning.

Change-Id: I16561901edc03467df5ebe783e60c9c096665258
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1283285
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>