Update mncc.h, but to not have to figure out ctypes generation and still be
MNCC v6 compatible, simply add a single SDP byte of '\0' to sent buffers, so
that an MNCC v6 MSC doesn't complain about a too short MNCC message.
Sending no SDP is compliant, and sending a single '\0' is the specified proper
way to do so, but we're achieving it in a hacky way.
To accept incoming MNCC v6: allow trailing data
Related: osmo-sip-connector I522ce7f206932a816a64f03d916799c3215bb8c7