Fix compile error on MacOS.

Summary:
In a release build, a member was not being accessed. This
member was only being accessed in a debug build. We now add an accessor
function for this member and the buid succeeds.

Test Plan: build release/unity/debug on linux/mac

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D34035

db/managed_iterator.cc

@@ -43,6 +43,7 @@ class MILock {
     XFUNC_TEST("managed_xftest_release", "managed_unlock", managed_unlock1,
                xf_manage_release, mi_);
   }
+  ManagedIterator* GetManagedIterator() { return mi_; }
 
  private:
   std::mutex* const mu_;