Support new versions of Bazel that don't provide cc_library by default

Recent versions of Bazel don't provide `cc_library` and friends by default. That causes an error if you try to link against google-cloud-cpp without `--incompatible_autoload_externally=true`:

```
File "/.../external/google_cloud_cpp+/BUILD.bazel", line 37, column 12, in
[cc_library(
File "/virtual_builtins_bzl/bazel/exports.bzl", line 40, column 9, in _removed_rule_failure
Error in fail:
This rule has been removed from Bazel. Please add a `load()` statement for it.
This can also be done automatically by running:
buildifier --lint=fix
```

I didn't see an existing tracking bug for this, so filing one now. (Feel free to dup, though, if I just missed it!)