community/xdg-desktop-portal: fix building docs

docs are only built when /usr/share/dbus-1/interfaces/org.freedesktop.Flatpak.xml is available. The package that provides this is 'flatpak', but that's not being pulled in by 'flatpak-dev', resulting in an empty -doc subpackage failing the build.

Add flatpak as a direct makedepends to fix this.