Reference types are non-POD

This is code that I'm sure never hits as:

1. A ``ref`` field is not permitted in D
2. The ``this`` field used to be marked as ``STCref``, however this is no more.

However, *if* someone decides that it might be a good idea to add ref fields to D, then it should not be ignored.  Reference types are not compatible with C structs, so mark it as non-POD and break.