Revert "lib: Fix building against old glib versions"

This reverts commit f1d9196076.

Since libglnx.h does not get installed, it can't be included in
ostree-autocleanups.h, which is included by ostree.h.

Closes: #1615
Approved by: jlebon

src/libostree/ostree-ref.c

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