fix: Use c_char instead of i8

[OlivierDehaene]

The following line is problematic as std::os::raw::c_char can either be u8 or i8 depending on the OS distribution.
I think it is possible to use c_char directly and avoid the issue.

What do you think?

[Enet4]

Although the implementation underneath does not depend on the code type, I suppose c_char is a more fitting match for what it's in faiss-sys, and it's a crate-only function anyway. Looks good to me. Thank you!