ostree-system-generator: Include <libglnx.h> for autocleanups

g_autoptr was new in GLib 2.44, but we officially only require 2.40,
so we need to use the backport in libglnx.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #1310
Approved by: cgwalters

src/switchroot/ostree-system-generator.c

@@ -25,6 +25,8 @@
 #include <fcntl.h>
 #include <stdlib.h>
 
+#include <libglnx.h>
+
 #include "ostree-cmdprivate.h"
 #include "ostree-mount-util.h"