roll: pigweed pw_async2: Codelab build fix

Step3 checkpoint2 added a use of a `std::lock_guard`, which requires
including `<mutex>`.

This patch updates the files past at and after this point to add the
missing include, and adjusts the step3 hint to call out the include as a
difference.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/326893
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: a9bda7e611f1b81b9742ebf8ec1fa315523b142b

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: b0069910339209..a9bda7e611f1b8
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8702384008562085281
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ibfb04871f2006b2638c08ccbd634ac120ff805e0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/showcase/sense/+/326933
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>