deps/rbdl: Add missing Eigen dependency

Fabian Reisterrequested to merge
fix/deps-rbdl-missing-dependency into main
  • RBDL module: adding missing dependency Eigen

Without this, the Eigen system version might be used instead (Ubuntu 22) or might lead to a failure on Ubuntu 18. See pipeline in Simox: https://git.h2t.iar.kit.edu/sw/simox/simox/-/pipelines/22179

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