leveldb: Register in copybara whitelist.

The documentation recommends modifying the whitelist after evaluating Copybara. However, evaluating requires significant workarounds without the whitelist entry. So, this CL adds leveldb to the whitelist early.

leveldb is currently open sourced to https://github.com/google/leveldb using MOE.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=240786286