Adds customized_form_field_behaviours.schema.json to MANIFEST.in (#13031)

The customized_form_field_behaviours.schema.json was present in
setup.cfg but missing in MANIFEST.in. Result of it was that
while it was present in .whl package, it was missing from sdist
package.

Fixes: #13027

MANIFEST.in

@@ -33,5 +33,6 @@ include airflow/alembic.ini
 include airflow/api_connexion/openapi/v1.yaml
 include airflow/git_version
 include airflow/provider.yaml.schema.json
+include airflow/customized_form_field_behaviours.schema.json
 include airflow/serialization/schema.json
 include airflow/utils/python_virtualenv_script.jinja2