sofia-sip allows applications to register a log backend function
which will be called every time the library wants to log something.
We register such a call-back and make it log using the libosmocore logging
framework.
The problem is that sofia-sip has its own log level management, and by
the time the message hits libosmocore, we don't know which log level we
shall use :(
Change-Id: Ib269b6b50f9d79bbd13acc43a626834921f05edb
Related: OS#3105