Fix compiler warning treated as error (#6547)

Summary:
Define a private member variable only in debug mode. Without fix, build will fail
```
In file included from table/block_based/partitioned_index_iterator.cc:9:
./table/block_based/partitioned_index_iterator.h:125:32: error: private field 'icomp_' is not used [-Werror,-Wunused-private-field]
  const InternalKeyComparator& icomp_;
```

Test plan (dev server)
1. make check
2. Make sure fixed in Travis
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6547

Reviewed By: siying

Differential Revision: D20480027

Pulled By: pdillinger

fbshipit-source-id: 288bc94280e240c3136335b6c73eb1ccb0db459d

table/block_based/partitioned_index_iterator.h

@@ -29,7 +29,9 @@ class ParititionedIndexIterator : public InternalIteratorBase<IndexValue> {
       TableReaderCaller caller, size_t compaction_readahead_size = 0)
       : table_(table),
         read_options_(read_options),
+#ifndef NDEBUG
         icomp_(icomp),
+#endif
         user_comparator_(icomp.user_comparator()),
         index_iter_(std::move(index_iter)),
         block_iter_points_to_real_block_(false),
@@ -122,7 +124,9 @@ class ParititionedIndexIterator : public InternalIteratorBase<IndexValue> {
  private:
   const BlockBasedTable* table_;
   const ReadOptions read_options_;
+#ifndef NDEBUG
   const InternalKeyComparator& icomp_;
+#endif
   UserComparatorWrapper user_comparator_;
   std::unique_ptr<InternalIteratorBase<IndexValue>> index_iter_;
   IndexBlockIter block_iter_;