[roll third_party/pigweed] pw_preprocessor: Switch to Doxygen

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/192730

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 8e87382862fa9df..a438bf7f3271870
Roller-URL: https://ci.chromium.org/b/8755530523168682833
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I275df3345c492ba56d0f28bd952e25ac9880ab8c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/193316
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 8e87382..a438bf7 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 8e87382862fa9df2f0d7de20af3e1c7acbcb236f
+Subproject commit a438bf7f3271870ff9e3a287590431bbfdc0ece2