fix issue when SOAP/wsdl calls are used

Without that android SOAP client (maybe others) don't work :-(

The error was : 

PHP message: PHP Fatal error:  require_once(): Failed opening required './core/http_api.php' (include_path='/XXX/htdocs/core/:/XXX/htdocs/library:../../library') in /XXX/htdocs/plugins/ThePoser/ThePoser.php on line 3