[roll third_party/pigweed] pw_protobuf_compiler: Add missing dep in CMake

Previously, pw_proto_library was missing a dependency on the
${NAME}._sources target. Proto builds would work if the ._sources target
had been run previously, but a clean build would fail.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/47361

third_party/pigweed Rolled-Commits: 4c05a061460b4bc..de26e4838f47817
Roller-URL: https://ci.chromium.org/b/8846033494478957456
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8846033494478957456
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic74f6c0b768cab81e0ffbd34a67e832efa3d28d7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/47381
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 4c05a06..de26e48 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4c05a061460b4bc63a3d340a6432cec2331e4fd9
+Subproject commit de26e4838f47817076b7d6c69d8d1f0f9bef60e0