Add visibility exemption for kythe C++ protoc plugin

This is similar to the exemption added to the python generator for gRPC.  We don't want to expose these internal implementations in general, but we also don't want to block the adopting of our main branch for existing uses within Google.

PiperOrigin-RevId: 485682767

src/google/protobuf/compiler/cpp/BUILD.bazel

@@ -79,6 +79,7 @@ cc_library(
     visibility = [
         "//pkg:__pkg__",
         "//src/google/protobuf/compiler:__pkg__",
+        "@io_kythe//kythe/cxx/tools:__subpackages__",
     ],
     deps = [
         ":names_internal",