Fix unused var warning in test

Author: lun3x

multi_index_map/tests/derive_serde.rs

@@ -22,5 +22,5 @@ fn should_compile() {
     };
     map.insert(elem1);
 
-    let s = serde_json::to_string(&map);
+    let _s = serde_json::to_string(&map);
 }