library/MNCC_EncDec.cc: fix encoding of optional 'sdp' field

Decoding was implemented in [1], but encoding was not.

Change-Id: I82f8afd3e64d83d165a7070a1b82a6276ef0b020
Fixes: [1] Ic9568c8927507e161aadfad1a4d20aa896d8ae30
1 file changed