Fix build with newest version of persistent-template.

This is untested because of rain, also I am operating from truncated
copiler error messages in a bug report that also doesn't mention what the
library version is. Still, it should work.

May break builds with old ghc, in particular DerivingStrategies is
I think fairly new? The pragmas could be ifdefed if necessary. Works with
ghc 8.6.5.

Database/Export.hs

@@ -10,6 +10,8 @@
 {-# LANGUAGE MultiParamTypeClasses, GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE StandaloneDeriving #-}
 
 module Database.Export (
 	ExportHandle,