Add git submodule init

[Dirreke]

This pull request enhances the build process by ensuring that required git submodules are initialized and updated before building, particularly when the build feature is enabled.

This helps in cases where the crate is obtained from a normal clone or through git = "..." in Cargo, where submodules may not have been fetched in advance. Without them, the native sources required by the build are unavailable, and the failure only shows up later during compilation.

For *-sys crates, handling bundled native sources in the build step is a common approach, since it makes the crate more self-contained and avoids extra setup requirements for downstream users.