Fix build with system libpng

build/linux/unbundle/libpng.gn

 # Copyright 2016 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_libpng") {
+pkg_config("libpng_config") {
   packages = [ "libpng" ]
 }
 
 shim_headers("libpng_shim") {
   root_path = "."
   headers = [
     "png.h",
     "pngconf.h",
   ]
 }
 
 source_set("libpng") {
   deps = [
     ":libpng_shim",
   ]
-  public_configs = [ ":system_libpng" ]
+  public_configs = [ ":libpng_config" ]
 }