Internal error for postulate in where block

Minimal example:
```agda
open import Haskell.Prelude

test : ⊤
test = local
  where postulate local : ⊤

{-# COMPILE AGDA2HS test #-}
```

Error message:
```
src/Agda2Hs/Compile/Function.hs:163:5-25: Non-exhaustive patterns in Function {..}
```

Should: compile the local definition as a postulate (i.e. a run-time error).