Fix build with ROCKSDB_LITE and -Wunused-private-field (#4715)

Summary:
The error message of databases/rocksdb-lite (FreeBSD port) is as follows:
```
  tools/db_bench_tool.cc:1976:16: error: private field 'trace_options_' is not used [-Werror,-Wunused-private-field]
    TraceOptions trace_options_;
                 ^
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4715

Differential Revision: D13207902

Pulled By: ajkr

fbshipit-source-id: be3c612eba656aeddb77e35e2f201dd25dc92f7e

tools/db_bench_tool.cc

@@ -1985,7 +1985,9 @@ class Benchmark {
   int64_t max_num_range_tombstones_;
   WriteOptions write_options_;
   Options open_options_;  // keep options around to properly destroy db later
+#ifndef ROCKSDB_LITE
   TraceOptions trace_options_;
+#endif
   int64_t reads_;
   int64_t deletes_;
   double read_random_exp_range_;