default.nix: add happy, which is needed by toml-parser.

default.nix

@@ -1,13 +1,14 @@
 # default.nix
-let 
+let
   pkgs = import <nixpkgs> { };
-in 
+in
   pkgs.haskellPackages.developPackage {
     root = ./.;
     modifier = drv:
       pkgs.haskell.lib.addBuildTools drv (with pkgs.haskellPackages;
         [ cabal-install
           ghcid
+          happy
         ]);
   }