[protobuf] Add repeated_ptr_field.cc to build

Needed since we upgraded to v3.19.4 so we don't get link errors.

Change-Id: Ia2bdda064e8034120cf8f24f0f1749f33b9850f6
diff --git a/BUILD.gn b/BUILD.gn
index 10acaf7..060d449 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -83,6 +83,7 @@
     "src/google/protobuf/message_lite.cc",
     "src/google/protobuf/parse_context.cc",
     "src/google/protobuf/repeated_field.cc",
+    "src/google/protobuf/repeated_ptr_field.cc",
     "src/google/protobuf/stubs/bytestream.cc",
     "src/google/protobuf/stubs/common.cc",
     "src/google/protobuf/stubs/int128.cc",