Fix unused variable failure (#7004)

Summary:
pass make check, run db_stress
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7004

Reviewed By: ajkr

Differential Revision: D22132617

Pulled By: zhichao-cao

fbshipit-source-id: d65397967e213206ec5efcb767bbdda8a575662a

db_stress_tool/db_stress_compaction_filter.h

@@ -36,6 +36,7 @@ class DbStressCompactionFilter : public CompactionFilter {
     uint64_t key_num = 0;
     bool ok = GetIntVal(key.ToString(), &key_num);
     assert(ok);
+    (void)ok;
     MutexLock key_lock(state_->GetMutexForKey(cf_id_, key_num));
     if (!state_->Exists(cf_id_, key_num)) {
       return Decision::kRemove;