net/nncp: Unbreak with go1.16

> Building for nncp-5.3.3

...
no required module provides package go.cypherpunks.ru/nncp/v5/cmd/nncp-cfgenc: working directory is not part of a module

PR: 249946
Approved by: Sergey Matveev <stargrave@stargrave.org> (maintainer)