Fix run_return example build error on non-desktop platforms (#1067)

examples/window_run_return.rs

@@ -1,11 +1,21 @@
-use winit::{
+// Limit this example to only compatible platforms.
+#[cfg(any(
+    target_os = "windows",
+    target_os = "macos",
+    target_os = "linux",
+    target_os = "dragonfly",
+    target_os = "freebsd",
+    target_os = "netbsd",
+    target_os = "openbsd"
+))]
+fn main() {
+    use winit::{
         event::{Event, WindowEvent},
         event_loop::{ControlFlow, EventLoop},
         platform::desktop::EventLoopExtDesktop,
         window::WindowBuilder,
-};
+    };
 
-fn main() {
     let mut event_loop = EventLoop::new();
 
     let window = WindowBuilder::new()
@@ -39,3 +49,8 @@ fn main() {
 
     println!("Okay we're done now for real.");
 }
+
+#[cfg(any(target_os = "ios", target_os = "android", target_os = "emscripten"))]
+fn main() {
+    println!("This platform doesn't support run_return.");
+}