Add atc binary to .gitignore.
Add atc binary to .gitignore.

[#93927268]

Signed-off-by: Rob Dimsdale <4df639ba78cac7f8d741d02ef508f2611d6911dd@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse