mgcp-client: Rearrange order of structs and APIs in header

Move mgcp_client_pool up in the file as a preparation for next commit,
where pool_member will have a pointer to the pool added.
The related APIs on the object are also moved up in the file.

Change-Id: I9ff9c6e1e722690835c5d59b1fa87fb7d9e3120c
1 file changed