added $data param to display_field method for EE 2.7.0 compatibility

EE 2.7.0 expects the $data parameter in order to be compatible with the parent class, otherwise you'll encounter the following error in the CP:

Fatal error: Declaration of Category_field_ft::display_field() must be compatible with that of EE_Fieldtype::display_field() in /Library/WebServer/Documents/shanghai/nyush/expressionengine/third_party/category_field/ft.category_field.php on line 229