Fix CUPS compilation error in OSX 10.12.

printing/backend/print_backend_cups.cc

 // Copyright (c) 2012 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.
 
 #include "printing/backend/print_backend_cups.h"
 
+#include <cups/ppd.h>
 #include <dlfcn.h>
 #include <errno.h>
 #include <pthread.h>
 
 #include <string>
 
 #include "base/debug/leak_annotations.h"
 #include "base/files/file_util.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
@@ skipping 268 matching lines @@
   // Use default (local) print server.
   if (print_server_url_.is_empty())
     return cupsGetNamedDest(CUPS_HTTP_DEFAULT, printer_name.c_str(), nullptr);
 
   HttpConnectionCUPS http(print_server_url_, cups_encryption_);
   http.SetBlocking(blocking_);
   return cupsGetNamedDest(http.http(), printer_name.c_str(), nullptr);
 }
 
 }  // namespace printing