client: move some items to internal header

There is an upcoming modification of structs mgcp_msg and mgcp_response.
When it is public API, it needs to be kept backwards compatible.

But no libosmo-mgcp-client caller (in the osmocom-cni world) has used
these structs in years and years. Everyone should use the higher level
osmo_mgcpc_* API only.

Move this legacy API to a private header, so we no longer need to worry
about compatibility there.

Related: OS#6171
Related: I798e02c6663376d3d52f4a74fc4b32411ce95bed
Change-Id: I6d8bdda5c2ffa236e94a0b6111de4dbfac923187
4 files changed