Exclude doc package from flatpak build
    
    The problem is that the BuildRequires ghostscript.spec in turn has a
    
      Requires:         %{_bindir}/dvips
    
    which would expand to non-existent /app/bin/dvips when libtommath is built as a
    flatpak (as a dependency of building libreoffice as a flatpak), which would
    break the build.  The easiest fix appears to be to just not build docs for the
    flatpak case.