Remove extra semi colon from faiss/impl/ProductAdditiveQuantizer.h

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

If the code compiles, this is safe to land.

Reviewed By: algoriddle

Differential Revision: D52969139

fbshipit-source-id: 563843d43c64fb988ff0d08ccdb0408cd0eba454

faiss/impl/ProductAdditiveQuantizer.h

@@ -151,4 +151,4 @@ struct ProductResidualQuantizer : ProductAdditiveQuantizer {
  ProductResidualQuantizer();
 };
 
-}; // namespace faiss
+} // namespace faiss