roll: third_party/pigweed pw_protobuf: Allow building with -Wsign-conversion

Original-Bug: 259746255
Original-Fixes: 479353024
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/369099
Original-Revision: 1540c703e635ef668d7d14036a8a2aaa6bd1c1e2

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: a9ef4f13684497..1540c703e635ef
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8690929553170038577
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id03ff1334b22df9e1efa5aefa336496f68c81f39
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/370832
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index a9ef4f1..1540c70 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit a9ef4f13684497ca35e1e7e88e3c7c6a5fd095de
+Subproject commit 1540c703e635ef668d7d14036a8a2aaa6bd1c1e2