Changed target_os from "win32" to "windows"

As of rust pull request 16435 (https://github.com/rust-lang/rust/pull/16435), cfg attributes now use "windows" instead of "win32" for the target_os parameter.

This caused linking against rust-sdl2 on Windows to fail since the #[link(name="SDL2")] attribute was ignored due to the mismatching cfg above it.