windows: os::fs requires default

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Author: Keruspe

src/os/windows/mod.rs

@@ -5,5 +5,6 @@ cfg_std! {
 }
 
 cfg_unstable! {
+    #[cfg(feature = "default")]
     pub mod fs;
 }