WAF: add obj.includes = '.' to command waf to do dependency tracking on private header files.

src/node/wscript

@@ -22,6 +22,7 @@ def build(bld):
         'ipv4.cc',
         'application.cc',
         ]
+    node.includes = '.'
 
     headers = bld.create_obj('ns3header')
     headers.source = [