osmo_ortp.c: Fix warning: implicit use of function
osmo_ortp.c: Fix warning: implicit use of function

Explicitly state that we are using an internal API from ortp, and
define the function we are using to avoid printing a warning.

Change-Id: I9cadcb31ce7ade3632d83753be97fdc9ea518b5b

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