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

util/options.cc

@@ -154,7 +154,8 @@ ColumnFamilyOptions::ColumnFamilyOptions(const Options& options)
       max_successive_merges(options.max_successive_merges),
       min_partial_merge_operands(options.min_partial_merge_operands) {
   assert(memtable_factory.get() != nullptr);
-  if (max_bytes_for_level_multiplier_additional.size() < num_levels) {
+  if (max_bytes_for_level_multiplier_additional.size() <
+      static_cast<unsigned int>(num_levels)) {
     max_bytes_for_level_multiplier_additional.resize(num_levels, 1);
   }
 }