can_unlock set but not used

Test Plan: I couldn't repro, but I hope this fixes it. See the error here: https://evergreen.mongodb.com/task_log_raw/mongodb_mongo_master_ubuntu1404_rocksdb_compile_6e9fd902d5cb25aef992363efa128640affd5196_16_02_11_04_33_37/0?type=T

Reviewers: yhchiang, andrewkr, sdong, anthony

Reviewed By: anthony

Subscribers: meyering, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D54123

utilities/transactions/transaction_base.cc

@@ -502,7 +502,7 @@ void TransactionBaseImpl::UndoGetForUpdate(ColumnFamilyHandle* column_family,
   auto& cf_tracked_keys = tracked_keys_[column_family_id];
   std::string key_str = key.ToString();
   bool can_decrement = false;
-  bool can_unlock = false;
+  bool can_unlock __attribute__((unused)) = false;
 
   if (save_points_ != nullptr && !save_points_->empty()) {
     // Check if this key was fetched ForUpdate in this SavePoint