Fix unused variable warning

Summary:
My last diff introduced a warning when compiling under release mode
https://reviews.facebook.net/D55539

fix the warning

Test Plan:
DEBUG_LEVEL=0 make db_bench
make check

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56295

utilities/write_batch_with_index/write_batch_with_index.cc

@@ -459,7 +459,7 @@ void WriteBatchWithIndex::Rep::AddNewEntry(uint32_t column_family_id) {
                           wb_data.size() - last_entry_offset);
   // Extract key
   Slice key;
-  bool success =
+  bool success __attribute__((__unused__)) =
       ReadKeyFromWriteBatchEntry(&entry_ptr, &key, column_family_id != 0);
   assert(success);