core: change import of exchange_alloc for win32 os::_list_dir

Author: olsonjeffery

src/libcore/os.rs

@@ -715,7 +715,7 @@ pub fn list_dir(p: &Path) -> ~[~str] {
             use os::win32::{
                 as_utf16_p
             };
-            use private::exchange_alloc::{malloc_raw, free_raw};
+            use unstable::exchange_alloc::{malloc_raw, free_raw};
             #[nolink]
             extern mod rustrt {
                 unsafe fn rust_list_dir_wfd_size() -> libc::size_t;