ipa: Add function declaration for IPA header creation
ipa: Add function declaration for IPA header creation

These functions are called from OpenBSC and are supposed to be
public API of libosmo-abis.

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