THRIFT-2767 hs: Transport.Empty is not externally available

Patch: Koen De Keyser

This Closes: #223
diff --git a/lib/hs/Thrift.cabal b/lib/hs/Thrift.cabal
index 7c9a295..9418c04 100755
--- a/lib/hs/Thrift.cabal
+++ b/lib/hs/Thrift.cabal
@@ -54,6 +54,7 @@
     Thrift.Protocol.JSON,
     Thrift.Server,
     Thrift.Transport,
+    Thrift.Transport.Empty,
     Thrift.Transport.Framed,
     Thrift.Transport.Handle,
     Thrift.Transport.HttpClient,