Handle bindgen signedness on Windows

[waych]

When building on x64_64-pc-windows-msvc, the types for enums are by
default signed, resulting in many errors of the flavor:

expected `u32`, found `i32`

for types all enum value assignments that map enum types to those
exposed by russimp-sys.

[jkvargas]

hey mate, thanks so much for your contribution.
Would you mind to raise the cargo.toml version please?
0.2.3 would be fine.

Also, I am looking for someone to add a windows building to our yml.
In case you interested haha =)

[waych]

FYI I got cargo test --lib building and passing on Windows locally. I should be able to send you a workflow config, but to make it work requires the following:

• a fix to the upstream vcpkg assimp port, as it seems to be linking two copies (incompatible?) of polyclipping
• small fix to russimp-sys to also link windows system libs (pulled in indirectly via irrlicht).

[jkvargas]

Sounds great bud.
I made a small request on your other PR.