[token-server] update gitignore

Summary: This dir is created when testing CI locally, e.g. ./contrib/teamcity/build-configurations.py token-server

Test Plan: Confirm impact is just .gitignore and we don't want this dir in the monorepo

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15618