docs: building locally impnotes fails

[Vladimir T. <vtz...@gm...>]

Trying to build locally impnotes gives following error:
vtz@vtz:~/clisp/dev/doc$ make html
make: *** No rule to make target `../modules/asdf/asdf.xml', needed by
`impnotes.xml'.  Stop.
There is no asdf/ in modules/ folder (not committed?).

Vladimir