ART: More header cleanups

Pull out more dependencies through forward declarations.

Test: m test-art-host
Change-Id: I7d86726928937f788b956ec9eac91532d66d57ae
diff --git a/runtime/monitor.cc b/runtime/monitor.cc
index f94edcd..a617818 100644
--- a/runtime/monitor.cc
+++ b/runtime/monitor.cc
@@ -31,6 +31,7 @@
 #include "lock_word-inl.h"
 #include "mirror/class-inl.h"
 #include "mirror/object-inl.h"
+#include "object_callbacks.h"
 #include "scoped_thread_state_change-inl.h"
 #include "stack.h"
 #include "thread.h"