Remove extra semi colon from faiss/IndexIVFFastScan.cpp

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

If the code compiles, this is safe to land.

Reviewed By: algoriddle

Differential Revision: D52969081

fbshipit-source-id: d728a4887ac1415ae88dae643055380a183c6990

faiss/IndexIVFFastScan.cpp

@@ -450,7 +450,7 @@ int compute_search_nslice(
     return nslice;
 }
 
-}; // namespace
+} // namespace
 
 void IndexIVFFastScan::search_dispatch_implem(
         idx_t n,