Fixed a compile error of cuckoo_table_builder.

Summary:
Fixed the following compile error.

./table/cuckoo_table_builder.h:72:22: error: private field 'key_length_' is not used [-Werror,-Wunused-private-field]
  const unsigned int key_length_;
                     ^
1 error generated.

Test Plan: make

Reviewers: sdong, ljin, radheshyamb, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20349