Add Zlib license to sdl2-sys (SDL 2's license)

[maia-s]

SDL 2 is licensed under the Zlib license, and I think the sys crate should reflect this. It'd be best if you could change the license of sdl2-sys to Zlib, but if this isn't possible it should at least be changed to "MIT AND Zlib". This would reflect the actual licenses involved, give proper credit to SDL, and make cargo tools that list licenses more accurate.

[Cobrand]

I'm not against the idea of having "MIT AND Zlib", a pull request is welcome. I am against just changing it to Zlib, as this isn't really accurate.

[maia-s]

Thank you, I've created the pull request. I do think it's kind of unfortunate that pure language bindings have a less liberal license than the library they're for though.