Fix a clang warning with GHC / Haskell Platform on Mac OS X 10.9

If you’re using GHC on Mac OS X 10.9 Mavericks with the ghc-clang-wrapper your terminal is probably spamming you with

warning: control reaches end of non-void function [-Wreturn-type]

Thankfully this is easy to fix, just edit /usr/bin/ghc-clang-wrapper and change line 30 from:

extraClangArgs="-Wno-invalid-pp-token -Wno-unicode -Wno-trigraphs"

to:

extraClangArgs="-Wno-invalid-pp-token -Wno-unicode -Wno-trigraphs -Wno-return-type"

Presumably this will be incorporated upstream eventually but for now this will take care of it.