Fix mysql feature also needs futures (#598)

crates/store/Cargo.toml

@@ -55,7 +55,7 @@ rocks = ["rocksdb", "rayon", "num_cpus"]
 sqlite = ["rusqlite", "rayon", "r2d2", "num_cpus", "lru-cache"]
 postgres = ["tokio-postgres", "deadpool-postgres", "tokio-rustls", "rustls", "ring", "rustls-pki-types", "futures", "bytes"]
 elastic = ["elasticsearch", "serde_json"]
-mysql = ["mysql_async"]
+mysql = ["mysql_async", "futures"]
 s3 = ["rust-s3"]
 foundation = ["foundationdb", "futures"]
 fdb-chunked-bm = []