[roll pigweed] pw_build: Append polyfill to public_deps

If default_public_deps is a non-empty list and
pw_span_ENABLE_STD_SPAN_POLYFILL is set to true, gn generation will fail
due to a non-empty public_deps list being overwritten. This CL fixes
that issue by appending to public_deps rather than setting.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/104460

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: 1d85cfee0e4d08e..b0dc7a0202e8a71
Roller-URL: https://ci.chromium.org/b/8806998308254296737
Cq-Cl-Tag: roller-builder:pigweed-zephyr-roller
Cq-Cl-Tag: roller-bid:8806998308254296737
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id4df4264eb9b7f332452aa0bf27cf6991f567c6c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/104462
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pigweed b/pigweed
index 1d85cfe..b0dc7a0 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit 1d85cfee0e4d08e7749eaeadf4e270c93e5e6eaf
+Subproject commit b0dc7a0202e8a710ba4e14a36fe8994619a67da8