protozero: Reduce binary size of protozero gen

This reduces the size of the generated ParseFromArray and Serialize
methods of generated protobuf message (the gen messages, not the
protozero message).

Before:
  stripped/example_system_wide size: 708304
After:
  stripped/example_system_wide size: 664816

Change-Id: I1583057af5d96094f776ab05d609144c89eaa36c
diff --git a/Android.bp b/Android.bp
index eadd256..946816b 100644
--- a/Android.bp
+++ b/Android.bp
@@ -8849,6 +8849,7 @@
     name: "perfetto_src_protozero_protozero",
     srcs: [
         "src/protozero/field.cc",
+        "src/protozero/gen_field_helpers.cc",
         "src/protozero/message.cc",
         "src/protozero/message_arena.cc",
         "src/protozero/message_handle.cc",