fix(runtime): fix pluggable_async_example feature gates

Change module-level #![cfg(feature = "async-api")] to item-level
#[cfg(feature = "async-api")] attributes to allow the fallback
main() function to be visible when the feature is disabled.

Previously, the entire module was gated causing the compiler to not
see any main() function, resulting in E0601 error.

With item-level attributes, the fallback main() on lines 117-121 is
now properly visible when async-api feature is not enabled.

Fixes: E0601 missing main function error
Related: #121

Author: avrabe

wrt-runtime/examples/pluggable_async_example.rs

@@ -5,8 +5,7 @@
 //! 2. Working with basic async/await patterns
 //! 3. Integration patterns for async code
 
-#![cfg(feature = "async-api")]
-
+#[cfg(feature = "async-api")]
 use core::{
     future::Future,
     marker::Unpin,
@@ -17,29 +16,33 @@ use core::{
     },
 };
 
+#[cfg(feature = "async-api")]
 use wrt_foundation::{
     is_using_fallback,
     with_async,
     AsyncRuntime,
     ExecutorError,
 };
 
-#[cfg(feature = "std")]
+#[cfg(all(feature = "async-api", feature = "std"))]
 extern crate alloc;
-#[cfg(feature = "std")]
+#[cfg(all(feature = "async-api", feature = "std"))]
 use std::boxed::Box;
 
 /// Simple async function for testing
+#[cfg(feature = "async-api")]
 async fn hello_async() -> &'static str {
     "Hello from simple async!"
 }
 
 /// Example future that is immediately ready
+#[cfg(feature = "async-api")]
 #[derive(Debug)]
 struct ReadyFuture {
     value: &'static str,
 }
 
+#[cfg(feature = "async-api")]
 impl Future for ReadyFuture {
     type Output = &'static str;
 
@@ -48,8 +51,10 @@ impl Future for ReadyFuture {
     }
 }
 
+#[cfg(feature = "async-api")]
 impl Unpin for ReadyFuture {}
 
+#[cfg(feature = "async-api")]
 fn main() {
     println!("=== Simple Async Executor Example ===\n");