Add Windows guard

rust/src/portals/screencast.rs

@@ -24,6 +24,8 @@ struct Runner;
 #[godot_api]
 impl INode for ScreencastRunner {
     fn init(base: Base<Node>) -> Self {
+        #[cfg(not(target_os = "linux"))]
+        panic!("Not supported on Windows");
         let (sender, receiver) = channel::unbounded();
         Self {
             base,