Fix compile failure on Travis

Summary:
Travis is complaining against using {} to initialize KVMap: https://travis-ci.org/facebook/rocksdb/jobs/84132600

      db/compaction_job_test.cc:526:26: error: chosen constructor is explicit in copy-initialization
        RunCompaction({files}, {});

This diff should fix it

Test Plan: travis

Reviewers: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D48309

db/compaction_job_test.cc

@@ -523,7 +523,9 @@ TEST_F(CompactionJobTest, FilterAllMergeOperands) {
 
   SetLastSequence(11U);
   auto files = cfd_->current()->storage_info()->LevelFiles(0);
-  RunCompaction({files}, {});
+
+  stl_wrappers::KVMap empty_map;
+  RunCompaction({files}, empty_map);
 }
 
 TEST_F(CompactionJobTest, SimpleSingleDelete) {