polkadot: enable tikv-jemallocator/unprefixed_malloc_on_supported_platforms (#2002)

This is indirectly enabled by rocksdb crate, better to make it explicit
(https://github.com/tikv/rust-rocksdb/blob/2096b9a161f93e437f7adee49e68cd1570aea42f/librocksdb_sys/Cargo.toml#L35-L38).