wiggle: fix compilation with async functions when tracing is off (#5203)

Fixes #5202

crates/wiggle/generate/src/funcs.rs

@@ -118,7 +118,11 @@ fn _define_func(
                 }.instrument(_span)
             )
         } else {
-            quote!(#body)
+            quote!(
+                async move {
+                    #body
+                }
+            )
         };
         (
             quote!(

crates/wiggle/tests/tracing.rs

@@ -0,0 +1,16 @@
+// This just tests that things compile when `tracing: false` is set,
+// which isn't the default.
+
+wiggle::from_witx!({
+    witx: ["$CARGO_MANIFEST_DIR/tests/atoms.witx"],
+    async: {
+        atoms::double_int_return_float,
+    },
+    tracing: false,
+});
+
+impl wiggle::GuestErrorType for types::Errno {
+    fn success() -> Self {
+        types::Errno::Ok
+    }
+}