tools/db_bench_tool.cc use uint64_t instead of size_t (#9800)

Summary:
to fix compilation for 32bit

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9800

Reviewed By: riversand963

Differential Revision: D35404447

fbshipit-source-id: 6a1185bb38f3a718357aa120e3b26a1ea77f023d

tools/db_bench_tool.cc

@@ -6397,9 +6397,9 @@ class Benchmark {
       }
 
       // Pick a Iterator to use
-      size_t db_idx_to_use =
+      uint64_t db_idx_to_use =
           (db_.db == nullptr)
-              ? (size_t{thread->rand.Next()} % multi_dbs_.size())
+              ? (uint64_t{thread->rand.Next()} % multi_dbs_.size())
               : 0;
       std::unique_ptr<Iterator> single_iter;
       Iterator* iter_to_use;