Fix windows builds with --features use-bindgen

[renski-dev]

The current master commit does not compile with --features use-bindgen. Specifically, here:

rust-sdl2/src/sdl2/keyboard/mod.rs

Line 194 in 0545cad

let arg = sys::SDL_Keymod(flags.bits() as u32);

The compiler complains:

error[E0308]: mismatched types
    --> src\sdl2\keyboard\mod.rs:197:35
     |
 194 |         let arg = sys::SDL_Keymod(flags.bits() as u32);
     |                   --------------- ^^^^^^^^^^^^^^^^^^^ expected `i32`, found `u32`
     |                   |
     |                   arguments to this struct are incorrect

Clang generates different integer types for C-style enums on different platforms, i32 on Windows and u32 on Linux. After some digging, there seems to be no neat way around this as far as I can tell, other than keeping this in mind whenever we care about the specific type of the underlying value.

More info: rust-lang/rust-bindgen#1966

Also somewhat related to #1511, which is the original reason why I'm building with use-bindgen in the first place.

[antonilol]

Thanks