Add missing build dependencies

services/python/content_handler/BUILD.gn

 # 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.
 
 import("//mojo/public/mojo_application.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//third_party/cython/rules.gni")
 
 mojo_native_application("content_handler") {
   output_name = "py_content_handler"
@@ skipping 41 matching lines @@
     "c_base.pxd",
     "$target_gen_dir/c_async_waiter.pxd",
     "$target_gen_dir/c_export.pxd",
     "mojo_system_impl.pyx",
   ]
   sources = [
     "python_system_impl_helper.cc",
     "python_system_impl_helper.h",
   ]
   deps = [
+    ":common_system_pxd",
     "//base",
     "//mojo/environment:chromium",
     "//mojo/common",
     "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/system",
     "//mojo/public/cpp/utility",
     "//mojo/public/cpp/bindings:callback",
     "//mojo/public/python:python_common",
   ]
 }