pubsub: add message_batch.h to build (#12744)

Author: scotthart

google/cloud/pubsub/CMakeLists.txt

@@ -75,6 +75,7 @@ add_library(
     internal/flow_controlled_publisher_connection.h
     internal/flow_controlled_publisher_tracing_connection.cc
     internal/flow_controlled_publisher_tracing_connection.h
+    internal/message_batch.h
     internal/noop_message_batch.h
     internal/ordering_key_publisher_connection.cc
     internal/ordering_key_publisher_connection.h

google/cloud/pubsub/google_cloud_cpp_pubsub.bzl

@@ -38,6 +38,7 @@ google_cloud_cpp_pubsub_hdrs = [
     "internal/extend_leases_with_retry.h",
     "internal/flow_controlled_publisher_connection.h",
     "internal/flow_controlled_publisher_tracing_connection.h",
+    "internal/message_batch.h",
     "internal/noop_message_batch.h",
     "internal/ordering_key_publisher_connection.h",
     "internal/publisher_auth_decorator.h",