GENEVA (Reuters) - U.S. singer Stevie Wonder will give a concert in Marrakesh on Friday, honoring his promise to perform if negotiators concluded an international treaty boosting access to books for blind and visually impaired people worldwide. The "Isn't She Lovely" star, who has been sightless since birth, lobbied hard for the pact approved on Thursday by more than 600 negotiators from 186 states, the World Intellectual Property Organization (WIPO) said in a statement. "This is a legacy, a gift to future generations. ...