[roll third_party/pigweed] pw_log_null: Missing public dependency and target

- Add missing public dependency dir_pw_preprocessor
- Add required target pw_log_null.impl

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48300

third_party/pigweed Rolled-Commits: ebbce4c898dad48..aa843845d42cc26
Roller-URL: https://ci.chromium.org/b/8844976750699943248
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8844976750699943248
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I4e4b38c923c5ea03ef072eb5ce7b33c5b63ae213
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/48341
Reviewed-by: 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 ebbce4c..aa84384 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit ebbce4c898dad4835f89374a8e84d4fe67aeb78d
+Subproject commit aa843845d42cc26fd951caf791a2ee3970460e56