[roll third_party/pigweed] pw_rpc: Fix double mutex unlock

The RPC lock was unlocked by both the lock guard and an unlock call.
This replaces the lock guard with a lock call.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/77304

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 89147b391efa687..16edd4074b2d6a5
Roller-URL: https://ci.chromium.org/b/8825735068169786385
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8825735068169786385
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I799f6c41bb17659fda8667a953e6a7f550356be5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/77441
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/third_party/pigweed b/third_party/pigweed
index 89147b3..16edd40 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 89147b391efa6878cbca1976f12c3f9e3d854214
+Subproject commit 16edd4074b2d6a5728deb7e6727ef056863339fd