See man git-add:
-f, --force
Allow adding otherwise ignored files.
So run this
git add --force my/ignore/file.foo
Despite Daniel Böhmer's working solution, Ohad Schneider offered a better solution in a comment:
If the file is usually ignored, and you force adding it - it can be accidentally ignored again in the future (like when the file is deleted, then a commit is made and the file is re-created.
You should just un-ignore it in the .gitignore file like that: Unignore subdirectories of ignored directories in Git
Another way of achieving it would be to temporary edit the gitignore file, add the file and then revert back the gitignore. A bit hacky i feel
Source: Stackoverflow.com