python-django-haystack: add missing 'setuptools' to propagatedBuildInputs

As a side-effect of f7e28bf5, the build
no longer propagated 'setuptools', which is a run-time dependency. See
https://github.com/NixOS/nixpkgs/pull/68314 for further details.

Fixes https://github.com/NixOS/nixpkgs/issues/68479.