fix #3777

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/muz/rel/dl_mk_explanations.h

@@ -66,6 +66,8 @@ namespace datalog {
         */
         mk_explanations(context & ctx);
 
+        ~mk_explanations() override;
+
         /**
            \brief Return explanation predicate that corresponds to \c orig_decl.
         */