Compile delta_generator and payload_consumer when "dbus" is disabled.

delta_generator and payload_consumer should not depend on any DBus
library. This patch now builds those even when "dbus" is disabled.

Bug: 25773375
TEST=`mma` on a target with DBus disabled.

Change-Id: Ia44cb9eed847e6d2eeb43a12373929128ae67d1c
diff --git a/system_state.h b/system_state.h
index 15350bf..2ba0272 100644
--- a/system_state.h
+++ b/system_state.h
@@ -17,7 +17,11 @@
 #ifndef UPDATE_ENGINE_SYSTEM_STATE_H_
 #define UPDATE_ENGINE_SYSTEM_STATE_H_
 
-#include <power_manager/dbus-proxies.h>
+namespace org {
+namespace chromium {
+class PowerManagerProxyInterface;
+}  // namespace chromium
+}  // namespace org
 
 class MetricsLibraryInterface;