[llvm-readobj] Should declare `ListScope` for `verneed` entries.

Summary: YAML mappings require keys to be unique. See: https://yaml.org/spec/1.2/spec.html#id2764652

Reviewers: jhenderson, grimar, rupprecht, espindola, ruiu

Reviewed By: ruiu

Subscribers: ruiu, emaste, arichardson, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60438

llvm-svn: 358078