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-metalink.h

@@ -40,6 +40,7 @@ struct OstreeMetalinkClass
 {
   GObjectClass parent_class;
 };
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (OstreeMetalink, g_object_unref)
 
 GType   _ostree_metalink_get_type (void) G_GNUC_CONST;