Adding indirect authordep to Pod::Weaver Contributors plugin

`dzil test` and `dzil build` weren't able to run due to `Pod::Weaver::Section::Contributors` being missing.  This module is an indirect dependency for `Dist::Zilla` (since `Pod::Weaver` needs it) and thus needs to be mentioned in the Authordeps section of `dist.ini`.  This looks like the correct location to put this change, if not, please let me know how I should alter the PR so as to fit to your coding style.