put language directives in the cabal file

gbs-downloader.cabal

@@ -68,6 +68,7 @@ common language
     NamedFieldPuns
     OverloadedStrings
     RecordWildCards
+    ScopedTypeVariables
     TupleSections
 
 library

src/Tahoe/Download.hs

-{-# LANGUAGE RecordWildCards #-}
-{-# LANGUAGE ScopedTypeVariables #-}
-
 {- | A high-level interface to downloading share data as bytes from storage
  servers.
 -}