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-repo.c

@@ -28,6 +28,7 @@
 #include <gio/gunixinputstream.h>
 #include <gio/gunixoutputstream.h>
 #include <gio/gfiledescriptorbased.h>
+#include "libglnx.h"
 #include "otutil.h"
 #include <glnx-console.h>