deskutils/xdg-desktop-portal: add missing dependency after 689ee986fad8

meson.build:115:18: ERROR: Dependency "gstreamer-pbutils-1.0" not found, tried pkgconfig

Found via multimedia/pipewire built with GSTREAMER option disabled.
See also https://github.com/flatpak/xdg-desktop-portal/commit/284279b35a28

(cherry picked from commit 72cdf9326eb207987f940350c804700a3de75afe)