Expose SeekData and SeekHole on all Linux targets
Expose SeekData and SeekHole on all Linux targets

These were previously missing on musl and mips targets because of
missing definitions in the libc crate.

geofft/nix-rust,geofft/nix-rust,nix-rust/nix,carllerche/nix-rust,asomers/nix,geofft/nix-rust,nix-rust/nix,carllerche/nix-rust,asomers/nix,carllerche/nix-rust