Make sure the NLPID appears in the protocol tree.

For protocols for which the NLPID is not considered part of the PDU, put
the NLPID into the protocol tree ourselves.

Bug: 10706
Change-Id: I2676d1934dd468ec755a6f94d95a23f025ba0f25
Reviewed-on: https://code.wireshark.org/review/5358
Reviewed-by: Guy Harris <guy@alum.mit.edu>