make faster_whisper.assets as a valid python package to distribute (#772

)

faster_whisper use `find_packages` in setup.py to scan directories and
distribute valid python packages. According to
https://setuptools.pypa.io/en/latest/userguide/package_discovery.html,
the `find_packages` function will ignore `faster_whisper/assets` which
is not a valid python package because it does not contain `__init__.py`.

However, when I build the package, setuptools raise a waring as #772,
and I do not know why. So I add `__init__.py` under the
`faster_whisper.assets` directory, making setuptools regard
`faster_whisper.assets` is a part of distribution.