Update to 0.9.21.1. Changes:
- Fix an assertion warning upon sensor start in case the address
for the local machine could not be found.
- Consistency rework of EasyBindings IDMEFCriteria API.
- Add refcount support for prelude_client_t and
prelude_client_profile_t, and update EasyBindings destructor to use
them.
- Fix a bug where EasyBindings would be built although they were not
enabled.
- Fix path issue in case libprelude was configured with specific path
outside of $prefix (fix #319).

(shannonjr)