make Callback use a separate empty type

SConstruct

@@ -61,6 +61,7 @@ core.add_headers ([
 ])
 core.add_inst_headers([
     'system-wall-clock-ms.h',
+    'empty.h',
     'callback.h',
     'ptr.h',
     'object.h',

src/core/callback.h

@@ -24,6 +24,7 @@
 
 #include "ptr.h"
 #include "fatal-error.h"
+#include "empty.h"
 
 namespace ns3 {
 
@@ -56,8 +57,6 @@ namespace ns3 {
  * and relies on a reference list rather than autoPtr to hold
  * the pointer.
  */
-class empty {};
-
 template <typename T>
 struct CallbackTraits;
 

src/core/empty.h

@@ -0,0 +1,8 @@
+#ifndef EMPTY_H
+#define EMPTY_H
+
+namespace ns3 {
+class empty {};
+}
+
+#endif /* EMPTY_H */