Fix build with old glib

We failed to include libglnx headers for the proxy which made
things fail if the autoptr support was not in glib.

xdg-app-proxy.c

@@ -18,6 +18,8 @@
  *       Alexander Larsson <alexl@redhat.com>
  */
 
+#include "config.h"
+
 #include <unistd.h>
 #include <string.h>
 

xdg-app-proxy.h

@@ -22,6 +22,7 @@
 #define __XDG_APP_PROXY_H__
 
 #include <gio/gio.h>
+#include "libglnx/libglnx.h"
 
 typedef enum {
   XDG_APP_POLICY_NONE,