add "multi" in @EXPORT_OK to export it with "-all", as is documented

First, thank you so much for Kavorka. I love the implicit use of `dwim_type` and the `multi`s.

Speaking of which: [The documentation](https://metacpan.org/pod/Kavorka#Exports) says that `use Kavorka -all;` will export (among others) the `multi` keyword, but `multi` wasn't mentioned in `@EXPORT_OK` so that didn't work and you had to export `multi` explicitly.

This commit just adds `multi` into `@EXPORT_OK`, sticking to the documentation.