Add gitignore of *_pb2.py to dashboard/dashboard.
Add gitignore of *_pb2.py to dashboard/dashboard.

This fixes untracked files due to generation of sheriff_config_pb2.py and
sheriff_pb2.py in dashboard/dashboard, which was added by
https://chromium-review.googlesource.com/c/catapult/+/1722641.

Change-Id: I641f2eed4793ac78e7089ac1dcc979020724783b
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/1947843
Auto-Submit: Andrew Bennetts <f07c55753dac44d78e0294a1d64ea321baaa78f4@google.com>
Reviewed-by: Dean Berris <34a37f59e4ade3cf49221306f04217acf3e86255@chromium.org>
Commit-Queue: Dean Berris <34a37f59e4ade3cf49221306f04217acf3e86255@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult