compilation fix for zenoh-c (#1776)

* followup to #1571: reliablity, sourceinfo to unstable

* typo fix to pass CI

* compilation fix

Author: milyin

zenoh/src/api/query.rs

@@ -161,6 +161,7 @@ impl Reply {
     pub unsafe fn empty() -> Self {
         Reply {
             result: Ok(Sample::empty()),
+            #[cfg(feature = "unstable")]
             replier_id: None,
         }
     }