Fix tests with serde1 feature

src/rngs/mock.rs

@@ -75,6 +75,8 @@ mod tests {
     #[test]
     #[cfg(feature = "serde1")]
     fn test_serialization_step_rng() {
+        use super::StepRng;
+
         let some_rng = StepRng::new(42, 7);
         let de_some_rng: StepRng =
             bincode::deserialize(&bincode::serialize(&some_rng).unwrap()).unwrap();