tree-wide: Use g_autoptr(Ostree*)

Part of cleaning up our usage of libglnx; we want to use what's in GLib where we
can.

Had to change a few .c files to `#include ostree.h` early on to pick up
autoptrs for the core types.

Closes: #1040
Approved by: jlebon

src/libostree/ostree-tls-cert-interaction.h

@@ -31,6 +31,7 @@ G_BEGIN_DECLS
 
 typedef struct _OstreeTlsCertInteraction        OstreeTlsCertInteraction;
 typedef struct _OstreeTlsCertInteractionClass   OstreeTlsCertInteractionClass;
+G_DEFINE_AUTOPTR_CLEANUP_FUNC(OstreeTlsCertInteraction, g_object_unref)
 
 GType                       _ostree_tls_cert_interaction_get_type    (void) G_GNUC_CONST;