Fix build with protobuf 6.30

Index: third-party/nanopb/generator/nanopb_generator.py
--- third-party/nanopb/generator/nanopb_generator.py.orig
+++ third-party/nanopb/generator/nanopb_generator.py
@@ -15,6 +15,7 @@ import tempfile
 import shutil
 import os
 from functools import reduce
+from google.protobuf import message_factory
 
 try:
     # Add some dummy imports to keep packaging tools happy.
@@ -1455,7 +1456,7 @@ class Message(ProtoElement):
         optional_only.name += str(id(self))
 
         desc = google.protobuf.descriptor.MakeDescriptor(optional_only)
-        msg = reflection.MakeClass(desc)()
+        msg = message_factory.GetMessageClass(desc)()
 
         for field in optional_only.field:
             if field.type == FieldD.TYPE_STRING:
