[meson] better propagate dependencies

The emper header LockedUnboundedQueue.hpp now depends on pthread.
To link libpthread with everything including LockedUnboundedQueue.hpp
we propagate emper_dependencies through emper_dep.
And using emper_dep as a dependency seems anyway better than essentially
writing down emper_dep manually each time (link_with:emper / include_directories: emper_all_include)