Fix dead code warnings (#373)

examples/customize-extractor-error/src/main.rs

@@ -41,6 +41,7 @@ async fn handler(Json(user): Json<User>) {
 }
 
 #[derive(Debug, Deserialize)]
+#[allow(dead_code)]
 struct User {
     id: i64,
     username: String,

examples/error-handling-and-dependency-injection/src/main.rs

@@ -147,6 +147,7 @@ struct User {
 }
 
 #[derive(Debug, Deserialize)]
+#[allow(dead_code)]
 struct CreateUser {
     username: String,
 }

examples/form/src/main.rs

@@ -55,6 +55,7 @@ async fn show_form() -> Html<&'static str> {
 }
 
 #[derive(Deserialize, Debug)]
+#[allow(dead_code)]
 struct Input {
     name: String,
     email: String,