Add overlay to bazel ignore
Add overlay to bazel ignore

Overlays shouldn't be touched even during analysis.
Tested:
Rebased on https://github.com/google/iree/pull/359
$ build_tools/scripts/bazel_build.sh
PiperOrigin-RevId: 287873454

google/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,google/iree,google/iree