Add Generators/GenInterfaces to the list of packages in AthSimulation

MR !4271 introduced a dependency on `Generators/GenInterfaces` in
the `AthSimulation` release, but missed adding it to the list of
packages to be built as part of the `AthSimulation` release.
This change should fix ATLASSIM-3427.


Former-commit-id: 4ff364ae0f285230214615be7f25a1ff6a6698bf