libsecret: update to 0.18.8.

Despite the changelog, the meson files are not included in
the distribution, so keep using autoconf for this release.

0.18.8
* Add support for g_autoptr() to our types [!11]
* Remove deprecated g_type_class_add_private() [!14]
* Bump GLib dependency (2.44+)
* Add meson build support [!9]
* Fix vapi generation [!15, ...]
* Build fixes [!12, !13]
* Updated translations

(wiz)