Fix Setup.hs

[alexfmpe]

$ git rev-parse HEAD
bc6729c5e7a7c9319c783ed945664f663d25116b

$ runhaskell Setup.hs 
Setup.hs:12:8: error:
    Variable not in scope: defaultMain
    Suggested fix:
      Perhaps you want to add ‘defaultMain’ to the import list
      in the import of ‘Distribution.Simple’ (Setup.hs:(4,1)-(5,57)).
   |
12 | main = defaultMain
   |        ^^^^^^^^^^^

Introduced in 93ccbe1

Doing cabal build works fine since build-type: Simple, but nixpkgs' haskell infra seems to insist on building it so I guess we might as well cater to it for the sake of non haskell.nix builds.

This is the smallest diff that seems to fix it, but maybe the whole Setup.hs should just be trimmed down instead ?