[third_party/pigweed] Roll 3 commits

6f6b6a12665eaac pw_transfer: Initial C++ client integration test
94552244f1913be pw_software_update: Add ManifestAccessor
3d2abe14b641ed4 pw_rpc: Only include <mutex> if locking is enabled

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: c6126af9a3d71b1..6f6b6a12665eaac
Roller-URL: https://ci.chromium.org/b/8827643836787588529
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8827643836787588529
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3367dac9471d9f1fc47e5b4b07d9fc31c74773b4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/75162
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 c6126af..6f6b6a1 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit c6126af9a3d71b185d8e2ec0cf50a910fc075481
+Subproject commit 6f6b6a12665eaac4854c6399e919736a9e32778c