Summary:
If you run `make test` on a non-Windows OS from a fresh build, the
testsuite will create an executable called `TimeMe` which `git` thinks is an
untracked file. Let's add it to `.gitignore` to avoid polluting the list of
untracked files.

Test Plan: Run `make test`, check if `TimeMe` is in the `git` untracked files

Reviewers: bgamari, austin

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3536