Fix build with system opus

build/linux/unbundle/opus.gn

 # Copyright 2017 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("//build/config/linux/pkg_config.gni")
 import("//build/shim_headers.gni")
 
-pkg_config("system_opus") {
+pkg_config("opus_config") {
   packages = [ "opus" ]
 }
 
 shim_headers("opus_shim") {
   root_path = "src/include"
   headers = [
     "opus.h",
     "opus_defines.h",
     "opus_multistream.h",
     "opus_types.h",
   ]
 }
 
 source_set("opus") {
   deps = [
     ":opus_shim",
   ]
-  public_configs = [ ":system_opus" ]
+  public_configs = [ ":opus_config" ]
 }
 
 source_set("opus_compare") {
 }
 
 source_set("opus_demo") {
 }
 
 source_set("test_opus_api") {
 }
 
 source_set("test_opus_decode") {
 }
 
 source_set("test_opus_encode") {
 }
 
 source_set("test_opus_padding") {
 }