The Debian build tools add all run-time dependencies automatically. For the
mariadb-rocksdb-plugin the "${misc:Depends}, ${shlibs:Depends}" will become
(based on the build dependencies at build time):

  Depends: mariadb-server-10.4 (= 1:10.4.12-1), rocksdb-tools,
  libc6 (>= 2.29), liblz4-1 (>= 0.0~r130), libsnappy1v5 (>= 1.1.8),
  libstdc++6 (>= 7), libzstd1 (>= 1.3.3), zlib1g (>= 1:1.1.4)

There is no need to manually add run-time build dependencies. Only external
dependencies need to be defined, for example rocksdb-tools could come in
question for mariadb-plugin-rocksdb, and it will be eventually added when
downstream official Debian packaging is merged upstream in a later commit.