Fix a compilation error in ROCKSDB_LITE in db/internal_stats.h

Summary:
Fix a compilation error in ROCKSDB_LITE in db/internal_stats.h

Other compilation errors will be fixed in a separate diff.

Test Plan: make OPT=-DROCKSDB_LITE

Reviewers: sdong, igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D36807

db/internal_stats.h

@@ -366,7 +366,7 @@ class InternalStats {
 
   void IncBytesMoved(int level, uint64_t amount) {}
 
-  void RecordLevelNSlowdown(int level, uint64_t micros, bool soft) {}
+  void RecordLevelNSlowdown(int level, bool soft) {}
 
   void AddCFStats(InternalCFStatsType type, uint64_t value) {}