Exclude systemd unit files for Flatpak builds
    
    While building gnome-maps RPM to construct Flatpak[0], we came
    across evolution-data-server installing systemd unit files. That
    isn't supported by Flatpak at the moment[1].
    
    The {?flatpak} macro is provided by flatpak-rpm-macros[2].
    
    Similar patches have been applied to other GNOME components that
    are being converted, such as:
    https://src.fedoraproject.org/rpms/spice-gtk/c/a6a6e6cd1e
    
    [0] https://docs.fedoraproject.org/en-US/flatpak/tutorial/
    [1] https://github.com/flatpak/flatpak/issues/2787
    [2] https://src.fedoraproject.org/rpms/flatpak-rpm-macros