Make threadt a struct

No function change.
As a convention we use class in presence of virtual function, which is
not the case here, so struct is prefered.

Author: romainbrenguier

src/goto-symex/goto_symex_state.h

@@ -349,9 +349,8 @@ class goto_symex_statet final
 
   unsigned total_vccs, remaining_vccs;
 
-  class threadt
+  struct threadt
   {
-  public:
     goto_programt::const_targett pc;
     guardt guard;
     call_stackt call_stack;