stunnel: fix SRC_URI

Trying to build stunnel fails with:

  ERROR 404: Not Found.

Seems that after the new release (5.x) they have moved the old tarball
under a 'archive/4.x' subdirectory.

Signed-off-by: Javier Viguera <javier.viguera@digi.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>