move dbus helper interface into its own file

This is step 1 to improving the interface to be more generic and drop
requirements on dbus.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I7835ea8242bfb7189cabc1a4084e5f84143d40f6
diff --git a/dbus/dbuspassive.hpp b/dbus/dbuspassive.hpp
index 74db80b..07a6ab2 100644
--- a/dbus/dbuspassive.hpp
+++ b/dbus/dbuspassive.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "conf.hpp"
+#include "dbushelper_interface.hpp"
 #include "dbuspassiveredundancy.hpp"
 #include "interfaces.hpp"
 #include "util.hpp"