[Bazel] ignore build files under libcxx's copy of google-benchmark

https://github.com/llvm/llvm-project/commit/5208ec5c66dc updated the
of google-benchmark under libcxx adding a BUILD.bazel file. This upsets
Bazel in our overlay configuration because it tries to interpret this
file.

Currently this is breaking the CI:
https://buildkite.com/llvm-project/upstream-bazel-rbe/builds/1828

Tested (same command the CI uses)
`bazel query //... + @llvm-project//... | xargs bazel test --config=generic-clang --config=rbe --test_output=errors --test_tag_filters=-nobuildkite --build_tag_filters=-nobuildkite`

Differential Revision: https://reviews.llvm.org/D105833