Build on older versions of glib

Various places need to include libglnx.h for the autoptr backport
fallbacks to be there before ostree-autocleanups.h is included.

This fixes the build on centos7·

Closes: #309
Approved by: giuseppe

src/libostree/ostree-fetcher.c

@@ -25,6 +25,7 @@
 #include <gio/gfiledescriptorbased.h>
 #include <gio/gunixoutputstream.h>
 
+#include "libglnx.h"
 #include "ostree-fetcher.h"
 #ifdef HAVE_LIBSOUP_CLIENT_CERTS
 #include "ostree-tls-cert-interaction.h"