main/postmarketos-ui-phosh: depend on xdg-desktop-portal-gtk

Add the dependency to properly support file transfer between sandboxed applications. Currently they can be used with flatpak (e.g. for quickly testing out applications, not a recommended way to applications) and long term we could use this in combination with apparmor for natively packaged applications.

Notably the KDE version is already getting pulled in by when installing postmarketos-ui-plasma-*.