protect typet::subtype() nonconst

This protects typet::subtype() nonconst.  There are no remaining uses of the
method outside of the typet class hierarchy.

Author: kroening

src/util/type.h

@@ -54,8 +54,6 @@ class typet:public irept
     return static_cast<const typet &>(get_sub().front());
   }
 
-  // This method will be protected eventually.
-public:
   typet &subtype()
   {
     subt &sub=get_sub();