Fixed a comparison between signed and unsigned integers in options.cc

Summary:
Fixed the following warning:

util/options.cc: In constructor ‘rocksdb::ColumnFamilyOptions::ColumnFamilyOptions(const rocksdb::Options&)’:
util/options.cc:157:58: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
   if (max_bytes_for_level_multiplier_additional.size() < num_levels) {
                                                             ^

Test Plan: make all check

Reviewers: haobo, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19293