configury: ensure ChangeLog even without gnulib_modules setting.

* bootstrap.slingshot (slingshot_ensure_changelog): New
function.  Hook bootstrap's func_ensure_changelog into
func_prep so that it works even when we don't have gnulib_modules
containing gitlog-to-changelog in our bootstrap.conf.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>