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.

Change-Id: I163ab1a5d765845abdcec6ec1cbcc984c1f146a8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/47361
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Alexei Frolov <frolv@google.com>