Bug#1016521: ristretto can't open .webp image file

[Akbarkhon Variskhanov]

Control: forwarded -1 https://gitlab.xfce.org/apps/ristretto/-/issues/50

As suggested in upstream[1], the `webp-pixbuf-loader` package enables
WebP support.

[1] https://gitlab.xfce.org/apps/ristretto/-/issues/50