fix(iso): used libc::Ioctl for FICLONE request type on musl targets

- musl defines ioctl's request parameter as c_int while glibc uses
  c_ulong; the hardcoded libc::c_ulong constant broke linux musl
  x64/arm64 native builds.

crates/pi-iso/src/linux_reflink.rs

@@ -80,7 +80,8 @@ mod imp {
 
 	use crate::{IsoError, IsoResult};
 
-	const FICLONE: libc::c_ulong = 0x4004_9409;
+	// `libc::Ioctl` is `c_int` on musl and `c_ulong` on glibc; the constant fits both.
+	const FICLONE: libc::Ioctl = 0x4004_9409;
 
 	pub fn start(lower: &Path, merged: &Path) -> IsoResult<()> {
 		let lower = canonical_existing_dir(lower)?;