ortp: make sure the ortp scheduler is started
ortp: make sure the ortp scheduler is started

the function osmo_rtp_init() is initializing ortp at the start of the
application. However, it does not start the scheduler of ortp, which
leads into error log messages in some situatios.

- make sure that ortp_scheduler_init() is called with osmo_rtp_init()

Change-Id: I3a63c23f5ede47773b6a249a48ecebd5d3b45ace
Related: OS#3299

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis