These were in some, but not all, of the existing BUILD files. They make
it easier to vendor cartographer, as Bazel complains if a BUILD file in
//third_party is missing a licenses() declaration.
This doesn't work when cartographer is an external workspace, due to a
limitation in cc_grpc_library. However, the //cartographer target will
still work it is external, and cartographer_grpc users can vendor the
repository.
Rodrigo Queiro