Changes

library: as_pfcp_ignore(): log SeqNr of received PDUs

Printing the PFCP PDU template ('?' by default) is not very informative
when reading logs.  Printing the message type of the received PDU is
not informative either, because message types are defined as numbers
in PFCP_Types.ttcn.  Printing the whole PDU is way too verbose, and
would be redundant given that the PFCPEM component already does print
all received PDUs.  Let's print the sequence number.

Change-Id: I803ff46def4ae0182310bc01e753fe0c05112836