devel/py-zipp: Add missing toml build dependency

A build issue occurs when devel/py-setuptools_scm has its TOML option
disabled.

PR: 256872
Reported by: Philipp Engel <kidon@posteo.de>
Approved by: portmgr (blanket)

(cherry picked from commit 8c08a2edf09b17f5fd1da63dc4c25f7eec8620b7)