[bzl] Fix Viewer builds on Mac

  bazel build //tools/viewer

Change-Id: I075b6aec33d87b315bd613d3b2ead73b16e91fdf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/937914
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
7 files changed