Also include the target generated directory in the includes search path

third_party/cython/rules.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 template("python_binary_source_set") {
   # Only available on linux for now.
   assert(is_linux)
   assert(defined(invoker.cython_sources) || defined(invoker.sources))
 
   config_name = target_name + "_python_config"
@@ skipping 11 matching lines @@
       visibility = target_visibility
       script = cython_script
       sources = invoker.cython_sources
       outputs = [
         cython_output,
       ]
       args = [
                "--cplus",
                "-I",
                rebase_path("//", root_build_dir),
+               "-I",
+               rebase_path(target_gen_dir, root_build_dir),
                "-o",
                rebase_path(cython_output, root_build_dir),
              ] + rebase_path(sources, root_build_dir)
     }
   }
 
   config(config_name) {
     visibility = target_visibility
     python_flags = "//third_party/cython/python_flags.py"
     include_dirs = exec_script(python_flags, [ "--includes" ], "list lines")
@@ skipping 108 matching lines @@
       "$root_out_dir/${shared_library_prefix}${shared_library_name}${shared_library_suffix}",
     ]
     outputs = [
       output,
     ]
     deps = [
       ":$shared_library_name",
     ]
   }
 }