Remove extra semi colon from faiss/python/python_callbacks.cpp

Summary:
`-Wextra-semi` or `-Wextra-semi-stmt`

If the code compiles, this is safe to land.

Reviewed By: algoriddle

Differential Revision: D52969076

fbshipit-source-id: 97692db6dd7a2d6ffef12ebcdaadd99abf089ca1

faiss/python/python_callbacks.cpp

@@ -22,7 +22,7 @@ struct PyThreadLock {
     }
 };
 
-}; // namespace
+} // namespace
 
 /***********************************************************
  * Callbacks for IO reader and writer