gprs_gb_parse: Add function to determine TLLI from encoded BSSGP

This will be needed to use the TLLI as link selector parameter
in osmo-gbproxy in an upcoming patch.

Depends: libosmocore.git I397b32a6e6ea3e9d218446138cceafa9b27685dd
Change-Id: Ia6d5300e63ad23987cbdca824db620305bd583d7