Add iproute2 to make "ss" available. It is used in the openbsc testsuite to dump socket statistics. If it isn't available, it does not cause tests to fail, but prints an error message that looks like it might be the real cause for the test failures:
> ss -tn /bin/sh: 1: ss: not found
I've also asked Harald if we should just retire the master-openbsc job, but we want to keep it around to ensure new libosmocore builds with this old codebase.
