jenkins: check for IU properly

The bug fixed by Idb034bf2ae749204836ab7e02018d769bb242a9f was not
detected by jenkins because it installs libosmo-sccp unconditionally.

Let's be more strict and install it only when necessary for IU builds to
make sure we can catch such bugs in future.

Change-Id: I573c7fec687edb71d79815b38a3bcf96273c03b3
1 file changed