Fix macvim compilation on 10.9 due to sigaltstack types mismatch.

Due to conflicting types for `sigaltstack` compilation fails. This updates the types used in os_unix.c. Relevant discussion in [vim-dev](https://groups.google.com/forum/#!msg/vim_dev/YHo51sA46fU/cybmVD3v3SkJ).

I'd guess this will be fixed upstream eventually. I have the same issue with vim, which I can also generate a patch for pretty easily.

My `brew doctor` and `brew install` is in [this gist](https://gist.github.com/zeekay/5954749).