style(p4, p9): fix style issue

problems/p9/src/main.rs

@@ -15,6 +15,7 @@ fn pkcs7_padding(data: &mut Vec<u8>, block_size: usize) {
     data.extend(vec![padding_length as u8; padding_length]);
 }
 
+#[allow(dead_code)]
 fn pkcs7_unpadding(input: &[u8]) -> Result<Vec<u8>> {
     if input.is_empty() {
         return Err(anyhow!("Input cannot be empty"));