The gsn_t changed the size with the addition of the ctrl pointer.
Bump the SO version to not break osmo-sgsn/ggsn on upgrade.
Call the -dev package libgtp-dev to follow the rest of Osmocom and
to ease making releases here.
Change-Id: Iac4d6d2effde1a6b2f60b1e1b49c91513d5ca8c3
Holger Hans Peter Freyther