make MTP3asp_PortType work without MTP3asp_PT.cc/hh

.. the latter depends on some third-party external library/code,
so let's make sure we can build it without that in the context of
a pure SIGTRAN stack.

Change-Id: I7f089c9cb1c0ccdc2c85f506fd8b48e0577ae1e5
Signed-off-by: Harald Welte <laforge@gnumonks.org>