added virtual destructor to statement_list_parse_treet::functiont

This re-enables compilation with clang.

Author: Daniel Kroening

src/statement-list/statement_list_parse_tree.h

@@ -170,6 +170,9 @@ class statement_list_parse_treet
     /// Prints the function in a human-readable form to the given output
     /// stream.
     virtual void output(std::ostream &out) const;
+
+    // destructor
+    virtual ~functiont() = default;
   };
   using functionst = std::list<functiont>;