Fix pdf_use_skia=1 on mac.

skia/ext/google_logging.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.
 
 // This file provides integration with Google-style "base/logging.h" assertions
 // for Skia SkASSERT. If you don't want this, you can link with another file
 // that provides integration with the logging of your choice.
 
+#include <stdarg.h>
 #include <stdio.h>
 
 #include "third_party/skia/include/core/SkTypes.h"
 
 void SkDebugf_FileLine(const char* file,
                        int line,
                        bool fatal,
                        const char* format,
                        ...) {
   va_list ap;
   va_start(ap, format);
 
   fprintf(stderr, "%s:%d ", file, line);
   vfprintf(stderr, format, ap);
   va_end(ap);
 }