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-repo-pull.c

@@ -22,6 +22,7 @@
 
 #include "config.h"
 
+#include "libglnx.h"
 #include "ostree.h"
 #include "ostree-core-private.h"
 #include "ostree-repo-private.h"