Add bazel-* directories in cargo_manifest_dir/external_crate to gitignore (#1279)

Previously running `bazel build ...` in examples/cargo_manfest_dir/external_crate
would leave the following untracked files:

    Untracked files:
      (use "git add <file>..." to include in what will be committed)
            examples/cargo_manifest_dir/external_crate/bazel-bin
            examples/cargo_manifest_dir/external_crate/bazel-external_crate
            examples/cargo_manifest_dir/external_crate/bazel-out
            examples/cargo_manifest_dir/external_crate/bazel-testlogs

I noticed that of the 5 workspaces found by `find . -name WORKSPACE.bazel`
three of them were ignoring "bazel-*" via the top-level .gitignore while
one had its own .gitignore containing only "bazel-*". I consolidated
that one into the top-level .gitignore and added the 5th.