Add static member definition

Author: ls4154

rocksdb/rocksdb_db.cc

@@ -113,6 +113,7 @@ namespace {
 
 namespace ycsbc {
 
+std::vector<rocksdb::ColumnFamilyHandle *> RocksdbDB::cf_handles_;
 rocksdb::DB *RocksdbDB::db_ = nullptr;
 int RocksdbDB::ref_cnt_ = 0;
 std::mutex RocksdbDB::mu_;