fix build for serde_support feature
diff --git a/src/rfc.rs b/src/rfc.rs
index d55bc74..7d8c319 100644
--- a/src/rfc.rs
+++ b/src/rfc.rs
@@ -2,6 +2,9 @@ use crate::Algorithm;
use crate::TotpUrlError;
use crate::TOTP;
+#[cfg(feature = "serde_support")]
+use serde::{Deserialize, Serialize};
+
/// Data is not compliant to [rfc-6238](https://tools.ietf.org/html/rfc6238)
#[derive(Debug, Eq, PartialEq)]
pub enum Rfc6238Error {