lib/sign: allow to build with glib version less than 2.44

Ubuntu 14.04 uses glib-2.40 which have no some shiny macroses
for interface declaration.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

src/libostree/ostree-sign-dummy.c

@@ -24,6 +24,7 @@
 
 #include "config.h"
 
+#include <libglnx.h>
 #include "ostree-sign-dummy.h"
 #include <string.h>
 
@@ -43,6 +44,10 @@ struct _OstreeSignDummy
   gchar *signature_ascii;
 };
 
+#ifdef G_DEFINE_AUTOPTR_CLEANUP_FUNC
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (OstreeSignDummy, g_object_unref)
+#endif
+
 static void
 ostree_sign_dummy_iface_init (OstreeSignInterface *self);