Implement thread#t_yield()

src/kernel/ucontext.rs

@@ -1,4 +1,4 @@
-
+#[cfg(not(target_os = "windows"))]
 use std::mem::MaybeUninit;
 
 /// Safe wrapper for ucontext_t struct of linux-gnu libc
@@ -54,7 +54,8 @@ impl UContextT {
     
 }
 
-#[cfg(target_os = "windows")] 
+#[cfg(target_os = "windows")]
+#[allow(unused)]
 impl UContextT {
 
     pub fn new() -> Self {