goto_statet cannot have a default constructor

It uses classes that don't have a default constructor.  This change is
required for compilation with clang 8.

Author: Daniel Kroening

src/goto-symex/goto_state.h

@@ -62,7 +62,7 @@ class goto_statet
   unsigned atomic_section_id = 0;
 
   /// Constructors
-  goto_statet() = default;
+  goto_statet() = delete;
   goto_statet &operator=(const goto_statet &other) = delete;
   goto_statet &operator=(goto_statet &&other) = default;
   goto_statet(const goto_statet &other) = default;