Make framet a struct

As convention we generaly use `class` in presence of virtual functions,
which is not the case here.

Author: romainbrenguier

src/goto-symex/goto_symex_state.h

@@ -267,9 +267,8 @@ class goto_symex_statet final
 
   // stack frames -- these are used for function calls and
   // for exceptions
-  class framet
+  struct framet
   {
-  public:
     // function calls
     irep_idt function_identifier;
     goto_state_mapt goto_state_map;