Fix clang pre-processor issue

I noticed this when trying to do `cabal install rest-core`. Similar to https://github.com/bos/aeson/issues/154, this fixes the following error when installing generic-aeson:

    Preprocessing library generic-aeson-0.1.0.3...

    src/Generics/Generic/Aeson.hs:14:4:
         error: invalid preprocessing directive
          #-}
           ^

I'm using

```
$ clang --version
Apple LLVM version 5.1 (clang-503.0.38) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
```