Add rust png build flags to gn_to_bp.py

BUG=502195264

Change-Id: I9611ca3b8060125e5d318fec05f1beffad340924
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1279536
Auto-Submit: Elliot Sisteron <elliotsisteron@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>