Add edm::reftobase::BaseHolder<*> type into a dictionary.

The following is needed for edm::RefToBase<*>, to store holder_
data member.

From Dr15Jones:
  holder_ must be stored since it holds all the information we need
  to retrieve the data. BaseHolder is an abstract base class but its
  concrete implementations ultimately hold an edm::Ref which must be
  stored

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>