Fix missing type syn::ItemFn in #[asyncs::test]

Author: kezhuw

asyncs-test/Cargo.toml

@@ -17,4 +17,4 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0.82"
 quote = "1.0.36"
-syn = "2.0.61"
+syn = { version = "2.0.61", features = ["full"] }