doc: mention python3-distutils dependency in BUILDING.md

Ran into #30189, solved by this comment:
https://github.com/nodejs/node/issues/30189#issuecomment-548250793

Refs: https://github.com/nodejs/node/issues/30189
PR-URL: https://github.com/nodejs/node/pull/33174


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Christian Clauss <cclauss@me.com>