Fix a missing async_trait annotation in bindgen! (#5747)

Closes #5743

Author: alexcrichton

crates/component-macro/tests/codegen/direct-import.wit

@@ -0,0 +1,3 @@
+default world foo {
+  import foo: func()
+}

crates/wit-bindgen/src/lib.rs

@@ -364,6 +364,9 @@ impl Wasmtime {
         }
 
         let world_camel = resolve.worlds[world].name.to_upper_camel_case();
+        if self.opts.async_ {
+            uwriteln!(self.src, "#[wasmtime::component::__internal::async_trait]")
+        }
         uwriteln!(self.src, "pub trait {world_camel}Imports {{");
         for sig in functions {
             self.src.push_str(sig);