tools: make mkssldef.py Python 3 compatible

This patch replaces the usage of `map` in such a way that it will be
compatible with Python 3.

PR-URL: https://github.com/nodejs/node/pull/25584


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>