Start moving stack-related stuff out of thread.h.

This was in preparation for implementing "SmartFrame", which is back on hold
while I finish finalization.

Change-Id: I006c9cfbc34520b9b0a72972611036d908a1d84b
diff --git a/src/object.cc b/src/object.cc
index 4950077..ce37470 100644
--- a/src/object.cc
+++ b/src/object.cc
@@ -19,6 +19,7 @@
 #include "logging.h"
 #include "monitor.h"
 #include "runtime.h"
+#include "stack.h"
 
 namespace art {