main/postmarketos-base-ui-gnome: add xdg-desktop-portal-gnome dep

xdg-desktop-portal-gnome is needed for example for mime type associations. Without it, the default gnome based shell installations couldn't detect which mime types were provided by which applications, xdg-open and the Default Applications tab in GNOME Control Center didn't work.

This dependency was missing from postmarketos-ui-phosh, so let's add it to postmarketos-base-ui-gnome to make sure it's not repeated in every gnome based UI.

@Newbyte