Makefile: remove the duplication of xml files for the docs
Makefile: remove the duplication of xml files for the docs

Use a variable substitution to generate the list so we don't have to
maintain them independently. Note that this removes the
FlatpakUpdateMonitor from the explicitly mentioned files but
gdbus-codegen seems to do the right thing with this now.

flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal