Add SDL_GetWindowWMInfo in sdl2_sys
Add SDL_GetWindowWMInfo in sdl2_sys

Because of the lack of untagged union support as of this commit's date,
this is not implemented in sdl2 yet.

Fixes #649

Cobrand/rust-sdl2,Cobrand/rust-sdl2,Rust-SDL2/rust-sdl2,Rust-SDL2/rust-sdl2,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,AngryLawyer/rust-sdl2