pw_tokenizer: List the setup.py in a BUILD.gn

pw_tokenizer now generates its setup.py since it includes a protobuf
module. Keep the original setup.py so pw_tokenizer can be installed
independently of GN. List this file in a BUILD.gn to satisfy the
source_is_in_build_files check.

Change-Id: I60631c8daf01f2928cb9c690c68b69531b0296d1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48440
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>