ART: More header cleanups

Pull out more dependencies through forward declarations.

Test: m test-art-host
Change-Id: I7d86726928937f788b956ec9eac91532d66d57ae
diff --git a/runtime/gc/reference_processor.cc b/runtime/gc/reference_processor.cc
index 886c950..52da763 100644
--- a/runtime/gc/reference_processor.cc
+++ b/runtime/gc/reference_processor.cc
@@ -22,6 +22,7 @@
 #include "mirror/class-inl.h"
 #include "mirror/object-inl.h"
 #include "mirror/reference-inl.h"
+#include "object_callbacks.h"
 #include "reference_processor-inl.h"
 #include "reflection.h"
 #include "ScopedLocalRef.h"