Compile libupdate_engine without DBus.

Test: mma with and without BRILLO_USE_DBUS=1
Test: adb shell /data/nativetest/update_engine_unittests/update_engine_unittests
Bug: 28800946

Change-Id: If3b05e7bc7a123d3d9b0dcc4597d915249a2de33
diff --git a/update_manager/real_system_provider.cc b/update_manager/real_system_provider.cc
index fde6bfa..44d5566 100644
--- a/update_manager/real_system_provider.cc
+++ b/update_manager/real_system_provider.cc
@@ -22,7 +22,9 @@
 #include <base/time/time.h>
 
 #include "update_engine/common/utils.h"
+#if USE_LIBCROS
 #include "update_engine/libcros_proxy.h"
+#endif
 #include "update_engine/update_manager/generic_variables.h"
 #include "update_engine/update_manager/variable.h"