gdbus: Fix compilation error due to missing #defines

Since these are simple #define strings, we are defining it here
instead of upgrading to D-Bus 1.5 or later.

  CC     gdbus/object.o
gdbus/object.c: In function ‘properties_set’:
gdbus/object.c:876:7: error: ‘DBUS_ERROR_UNKNOWN_PROPERTY’ undeclared
 (first use in this function)
gdbus/object.c:876:7: note: each undeclared identifier is reported
 only once for each function it appears in
gdbus/object.c:881:6: error: ‘DBUS_ERROR_PROPERTY_READ_ONLY’
 undeclared (first use in this function)
make[1]: *** [gdbus/object.o] Error 1
make: *** [all] Error 2
diff --git a/gdbus/object.c b/gdbus/object.c
index ec98c2c..214fd84 100644
--- a/gdbus/object.c
+++ b/gdbus/object.c
@@ -39,6 +39,14 @@
 #define DBUS_INTERFACE_OBJECT_MANAGER "org.freedesktop.DBus.ObjectManager"
+#define DBUS_ERROR_UNKNOWN_PROPERTY "org.freedesktop.DBus.Error.UnknownProperty"
+#define DBUS_ERROR_PROPERTY_READ_ONLY "org.freedesktop.DBus.Error.PropertyReadOnly"
 struct generic_data {
 	unsigned int refcount;
 	DBusConnection *conn;