Log Message:
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