Only try to build fd doctest on Unix

Author: sbrocket

src/extern_type.rs

@@ -63,6 +63,7 @@
 ///
 /// ```no_run
 /// // file/src/lib.rs
+/// # #[cfg(unix)]
 /// # mod file {
 /// # use std::os::unix::io::{IntoRawFd, FromRawFd};
 /// #[cxx::bridge(namespace = file::ffi)]