configure.ac: Require libosmocore 1.2.0
configure.ac: Require libosmocore 1.2.0

Some commits started using libosmocore APIs
osmo_gsm48_rest_octets_si3_encode() and lapdm_channel_init2(), which are
only available in latest libosmocore release 1.2.0. Let's update
configure.ac accordingly.

Fixes: 1e96e31c106aed9192ed9ea78ff1a711464390d7
Fixes: 46d62b984a10b3966d8a0d0a435d4efab0e9c500
Change-Id: If84c53d8192e8db98eb9439b898d83e94c5e5476

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