config: Install python3-dev package in buildroot

If the build needs to install a Python package from PyPI (like
hooks/image/60-kolibri-content), it's possible that it will need to be
built natively from source if there are no binary wheels available for
the current Python version. That fails if the Python development
environment isn't available.

https://phabricator.endlessm.com/T35125

config/defaults.ini

@@ -84,6 +84,7 @@ packages_add =
   pigz
   python3
   python3-apt
+  python3-dev
   python3-gi
   python3-requests
   python3-venv