The issue was reported @https://github.com/pentoo/pentoo-overlay/issues/62 originally. dnisff is the old software and the patch is trivial. It can be applied unconditionally.
Created attachment 422170 [details, diff] 2.4_beta1-libressl.patch
You probably meant https://github.com/pentoo/pentoo-overlay/issues/61
Support was added in net-analyzer/dsniff-2.4_beta1-r10