Making persistent-template compile with template-haskell 2.9

That with, with template-haskell 2.9 prior to this commit:
https://github.com/ghc/packages-template-haskell/commit/e4f742d5f70eefb1e0a7542762f34e7a50f6fcef#Language/Haskell/TH/Syntax.hs
Making it work with both TH2.8 and TH2.9