Rename object_bitmap to heap_bitmap (since that's what the class is called).

Change-Id: Idce6e9062545eb13b701e6b7e371c262977814d1
diff --git a/src/heap.h b/src/heap.h
index 18b521d..433bf6a 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -20,7 +20,7 @@
 #include <vector>
 
 #include "globals.h"
-#include "object_bitmap.h"
+#include "heap_bitmap.h"
 #include "offsets.h"
 
 #define VERIFY_OBJECT_ENABLED 0