VARISTOR symbol added

[Werner Almesberger]

?lvaro Lopes wrote:
> just added a varistor symbol with alias for KEV040209151A and
> KEV040205331A parts we use.

Thanks ! I've added it to the expanded view as well, on the audio
page. (That's the first place where varistors appear.)

> I think we might omit the pin number there. Please review as usual.

Looks great. And yes, it's better without the pin numbers since
it's non-polar anyway.

Thanks,
- Werner