sst_dump: Set dummy prefix extractor for binary search index in block based table

Summary: Now sst_dump fails in block based tables if binary search index is used, as it requires a prefix extractor. Add it.

Test Plan: Run it against such a file to make sure it fixes the problem.

Reviewers: yhchiang, kailiu

Reviewed By: kailiu

Subscribers: ljin, igor, dhruba, haobo, leveldb

Differential Revision: https://reviews.facebook.net/D18927