If GHC >= 7.8, module is always Thrustworthy due to GHC.Exts

Hi,

I was testing this module with -f-hashable, and I noticed it didn't build on GHC 7.8.2:

```
src/Data/List/NonEmpty.hs:142:1:
    GHC.Exts: Can't be safely imported! The module itself isn't safe.
```

This PR fixes this without further consequences (I hope).

Cheers