Update src/goto-symex/goto_symex_state.h

Co-Authored-By: smowton <[email protected]>

Author: tautschnig

src/goto-symex/goto_symex_state.h

@@ -230,7 +230,7 @@ class goto_symex_statet final
   void pop_frame() { call_stack().pop_back(); }
   const framet &previous_frame() { return *(--(--call_stack().end())); }
 
-  void print_backtrace(std::ostream &);
+  void print_backtrace(std::ostream &) const;
 
   // threads
   unsigned atomic_section_id;