Fix unused import on windows

test_suite/tests/test_ser.rs

@@ -4,10 +4,12 @@ extern crate serde_derive;
 use std::collections::{BTreeMap, HashMap, HashSet};
 use std::net;
 use std::path::{Path, PathBuf};
-use std::str;
 use std::time::Duration;
 use std::ffi::CString;
 
+#[cfg(unix)]
+use std::str;
+
 extern crate serde;
 
 extern crate serde_test;