[roll third_party/pigweed] pw_base64: Link test library on Zephyr test builds

Automatically link the test library to 'app' when building a test build
and setting the base64 Kconfig.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/148273

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: d9ed11f9d50d29b..56f9f4c41595d63
Roller-URL: https://ci.chromium.org/b/8780350107825914257
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ia9ad4b8a173ca8e53b4a9c2d6846009b67a10427
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/148437
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 d9ed11f..56f9f4c 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit d9ed11f9d50d29b9ca786f45e580cea7f649b14a
+Subproject commit 56f9f4c41595d6379a2283866b01b5a163f53fcd