truncate patch

Summary:
omit the override for the previous commit
Closes https://github.com/facebook/rocksdb/pull/1898

Differential Revision: D4598743

Pulled By: lightmark

fbshipit-source-id: f98a378

db/db_test_util.h

@@ -231,7 +231,7 @@ class SpecialEnv : public EnvWrapper {
           return base_->Append(data);
         }
       }
-      Status PositionedAppend(const Slice& data, uint64_t offset) {
+      Status PositionedAppend(const Slice& data, uint64_t offset) override {
         if (env_->table_write_callback_) {
           (*env_->table_write_callback_)();
         }