pw_snapshot: Narrow build dependency

Depending on a specific subtarget shrinks the build graph, and is
required internally.

Bug: b/218748512
Change-Id: I03a5c657a1ba8c8cf312e9093118196416b1fcd4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/100466
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Ted Pudlik <tpudlik@google.com>

pw_snapshot/BUILD.bazel

@@ -33,7 +33,7 @@ pw_cc_library(
     ],
     includes = ["public"],
     deps = [
-        ":metadata_proto_cc",
+        ":metadata_proto_cc.pwpb",
         "//pw_bytes",
         "//pw_protobuf",
         "//pw_status",