Add missing __init__.py.

init.py files are required to make Python treat directories containing the file as packages, and have them installed in the conda environment at build time.

I tested locally by creating conda-dist distribution with:

$ make env

differential-privacy is present in the generated archive:

$ tar xvzf dist/differential-privacy-0.1.0.conda-main-94a3603a.tgz -C venv
$ ls ./venv/lib/python3.7/site-packages/differential_privacy 
__init__.py   country_project_page_gaussian.py  utils/
__pycache__/  data_preparation.py