Use raw c_char to avoid issue on other archs

[jreidinger]

For details see rust-lang/rust-bindgen#1401

Problem

Build failed due to other archs using u8 instead of i8.

Solution

Use c_char so proper type is used on respective archs.

Testing

• test build is at https://build.opensuse.org/project/show/systemsmanagement:Agama:branches:fix_other_arch_build
• it is still failing, but now in test suite which is caused by different arch strings in product and in std::env at https://github.com/agama-project/agama/blob/api-v2/rust/agama-utils/src/products.rs#L180