Fix Cloud C++ build failure

Cloud C++ has a CI test that runs against protobuf head and this broke with one
of my recent changes. This change fixes the problem by restoring the previous
visibility entry that was changed.

I'm a little bit surprised that this works, since `@upb` doesn't even exist in
our workspace anymore. It appears that Bazel is happy to ignore that in our
workspace where it doesn't exist, and still respect it in other workspaces
where it does exist.

PiperOrigin-RevId: 565093972

src/google/protobuf/compiler/BUILD.bazel

@@ -200,7 +200,11 @@ filegroup(
 filegroup(
     name = "plugin_proto_src",
     srcs = ["plugin.proto"],
-    visibility = ["//upb:__subpackages__"],
+    visibility = [
+        "//upb:__subpackages__",
+        # For compatibility with workspaces that still use the old upb repo
+        "@upb//:__subpackages__",
+    ],
 )
 
 exports_files(