We don't really *need* it in libosmocore as such, but the lack of
having all osmocom extensions listed here lead to using overlapping
definitions: 0x18 was used for dynamic PDCH on the Abis side, but also
for CBCH on the L1SAP side. Let's list them all here to increase
visibility in case anyone wants to extend this further...
Related: OS#4027
Change-Id: I93e557358cf1c1b622f77f906959df7ca6d5cb12