Reduce header files including header files.

Main focus is getting heap.h out of runtime.h.

Change-Id: I8d13dce8512816db2820a27b24f5866cc871a04b
diff --git a/runtime/thread-inl.h b/runtime/thread-inl.h
index fc886d5..b1180bd 100644
--- a/runtime/thread-inl.h
+++ b/runtime/thread-inl.h
@@ -21,9 +21,11 @@
 
 #include <pthread.h>
 
+#include "cutils/atomic-inline.h"
+
 #include "base/casts.h"
 #include "base/mutex-inl.h"
-#include "cutils/atomic-inline.h"
+#include "gc/heap.h"
 #include "jni_internal.h"
 
 namespace art {