lib: Reexport libc::AT_FDCWD

Useful with `Repo::open_at()`.

Right now ostree-rs-ext pulls in libc for this and `fgetxattr`,
but the latter should go into nix.

rust-bindings/rust/src/lib.rs

@@ -16,6 +16,9 @@ pub use ffi;
 pub use gio;
 pub use glib;
 
+/// Useful with `Repo::open_at()`.
+pub use libc::AT_FDCWD;
+
 // code generated by gir
 #[rustfmt::skip]
 #[allow(clippy::all)]