Allow the HandoverPreparationInfoPDU dissector to be looked up by name and called.

Change-Id: I9c22b74d5c3329b6c7d74547d26e940c84ffc66c
Reviewed-on: https://code.wireshark.org/review/4247
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>