11.0/trisquel-icon-theme: add missing ayatana-settings icon for trisquel theme.

ubuntu-mate-default-settings defines a .desktop file that overrides the ayatana-settings one. So the ubuntu-mate can customize using their own icon through default ubuntu-mate themes.

Using the default ayatana-settings icon on the default trisquel one prevents a broken icon, when using trisquel themes (light or dark).