[roll third_party/pigweed] pw_uart: Add pw_uart to CMakeLists

Fix pw_uart import by adding to root CMakeLists.txt

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/210731
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 506466d72aa5cbf..83644e88fe7aa51
Roller-URL: https://ci.chromium.org/b/8747257523240905953
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I6724b9a3cb7adffce49f2cdab98a4740a274280c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/quickstart/bazel/+/210891
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 506466d..83644e8 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 506466d72aa5cbf02128d6e967bc2292f1a09b44
+Subproject commit 83644e88fe7aa518988670f67ffdf1cde26065cb