lib: Fix building against old glib versions

This commit includes libglnx.h in ostree-autocleanups.h, so we get the
g_autoptr backports wherever they're needed. Also, remove the "#include
libglnx.h" lines elsewhere that are no longer needed.

Closes: #1596
Approved by: cgwalters

src/libostree/ostree-ref.c

@@ -27,7 +27,6 @@
 #include <gio/gio.h>
 #include <glib.h>
 #include <glib-object.h>
-#include <libglnx.h>
 
 #include "ostree-autocleanups.h"
 #include "ostree-core.h"