Fix compile

Summary:
This commit broke the compile: 3ce3bb3da2
As evidenced here: https://evergreen.mongodb.com/task/mongodb_mongo_master_ubuntu1404_rocksdb_compile_ce2b1d11d42de93f7b375f7e6c41fb709f66e969_15_06_04_23_09_36

This should fix it

Test Plan: make check

Reviewers: IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D39627

db/version_set.cc

@@ -1221,10 +1221,10 @@ void VersionStorageInfo::GenerateLevel0NonOverlapping() {
       level_files_brief_[0].files,
       level_files_brief_[0].files + level_files_brief_[0].num_files);
   sort(level0_sorted_file.begin(), level0_sorted_file.end(),
-       [this](FdWithKeyRange& f1, FdWithKeyRange& f2) -> bool {
-         return (internal_comparator_->Compare(f1.smallest_key,
-                                               f2.smallest_key) < 0);
-       });
+       [this](const FdWithKeyRange & f1, const FdWithKeyRange & f2)->bool {
+    return (internal_comparator_->Compare(f1.smallest_key, f2.smallest_key) <
+            0);
+  });
 
   for (size_t i = 1; i < level0_sorted_file.size(); ++i) {
     FdWithKeyRange& f = level0_sorted_file[i];