oc2g: Add get_hwversion_desc to header file
oc2g: Add get_hwversion_desc to header file

it's used in oc2g/main.c and it needs to be in a header file.

Similar as previously done for lc15 in 19795c5a.

Change-Id: Ic6826d8c8ff5c648158493454a80704bb956b51d

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts