roll: third_party/pigweed pw_protobuf_compiler: Fix unused variable

use_legacy_oneof_interfaces is an optional variable and might not be
used. This triggers the GN "unused" error.

Original-Bug: 382416987
Test: Build pass
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/278332
Original-Revision: 2861b6ad5f94c2c8b4b393761d93c568eef9547e

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 3003d35d53be38..2861b6ad5f94c2
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8719461439728994561
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ib05abd5a62a61eb559c7d815bf01da0c4c41c677
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/278317
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.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 3003d35..2861b6a 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 3003d35d53be38d41a67907b59591edc8aeaf73d
+Subproject commit 2861b6ad5f94c2c8b4b393761d93c568eef9547e