Unused managed iterator

Summary: This causes warnings on OS X

Test Plan: compiles

Reviewers: rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D33969

db/managed_iterator.cc

@@ -46,7 +46,7 @@ class MILock {
 
  private:
   std::mutex* const mu_;
-  ManagedIterator* mi_;
+  ManagedIterator* mi_ __attribute__((unused));
   // No copying allowed
   MILock(const MILock&) = delete;
   void operator=(const MILock&) = delete;