Vanessa Williams is joining the third season of TNT’s “The Librarians,” Variety has learned. Williams joins in a recurring role, playing General Rockwell, the tough, brilliant head of the top secret, black ops secret government agency, known as D.O.S.A. Investigating “Statistical Anomalies,” Williams’ character sets her sights on apprehending the Librarians and obtaining the magical artifacts contained in the Library. The third season of TNT’s fan-favorite series will see the Librarians confronting a new antagonist, which has shown up in the form of D.O.S.A. — The Department of Statistical Anomalies — whose mission is to apprehend the Librarians and take control of the Library and all the magical artifacts hidden…