Changes

debian: add missing libsctp dependency

This dependency is *not* added automatically by `${shlibs:Depends}`
because it's not a dependency of `erlang-base`: it's listed as a
recommendation (in `Recommends`).  The `osmo-s1gw` package needs
to depend on libsctp explicitly;  otherwise it will not work
in a clean environment.

Change-Id: I9add00e501110b54447faf89fce0f18432663463