Fix BUILD.gn correctly this time.

Change-Id: Ic8ca9288523756ad52b83fdf8d3a6cbc255cce2f
diff --git a/BUILD.gn b/BUILD.gn
index 1712d85..5d22746 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -345,7 +345,7 @@
       "src/google/protobuf/compiler/cpp/cpp_message_field.cc",
       "src/google/protobuf/compiler/cpp/cpp_message_field.h",
       "src/google/protobuf/compiler/cpp/cpp_padding_optimizer.cc",
-      "src/google/protobuf/compiler/cpp/cpp_padding_optimizer.cc"
+      "src/google/protobuf/compiler/cpp/cpp_padding_optimizer.h",
       "src/google/protobuf/compiler/cpp/cpp_primitive_field.cc",
       "src/google/protobuf/compiler/cpp/cpp_primitive_field.h",
       "src/google/protobuf/compiler/cpp/cpp_service.cc",