Include requirement-setuptools.txt in manifest

When attempting to `pip install ckan` it fails as the setup.py requires
the requirement-setuptools.txt to be present, and it is not.  This
change makes sure that the pip install will not fail due to the absence
of this file.

This PR does not include the requirements.txt as this would likely
require further discussion about how requirements are loaded.

Fixes #4271

MANIFEST.in

@@ -19,4 +19,4 @@ include CHANGELOG.txt
 include ckan/migration/migrate.cfg
 include ckan/migration/README
 recursive-include ckan/migration/versions *.sql
-recursive-include ckan_deb *
+include requirement-setuptools.txt