Enable C linkage in fpdfppo.h

fpdfsdk/include/fpdfppo.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
  
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef _FPDFPPO_H_
 #define _FPDFPPO_H_
 
 #include "fpdfview.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 // Function: FPDF_ImportPages
 //                      Import some pages to a PDF document.
 // Parameters:  
 //                      dest_doc        -       The destination document which add the pages.
 //                      src_doc         -       A document to be imported.
 //                      pagerange       -       A page range string, Such as "1,3,5-7". 
 //                                                      If this parameter is NULL, it would import all pages in src_doc.
 //                      index           -       The page index wanted to insert from.   
 // Return value:
 //                      TRUE for succeed, FALSE for Failed.     
 DLLEXPORT FPDF_BOOL STDCALL FPDF_ImportPages(FPDF_DOCUMENT dest_doc,FPDF_DOCUMENT src_doc, FPDF_BYTESTRING pagerange, int index);
 
 
 // Function: FPDF_CopyViewerPreferences
 //                      Copy the viewer preferences from one PDF document to another.#endif
 // Parameters:  
 //                      dest_doc        -       Handle to document to write the viewer preferences to.
 //                      src_doc         -       Handle to document with the viewer preferences.
 // Return value:
 //                      TRUE for success, FALSE for failure.
 DLLEXPORT FPDF_BOOL STDCALL FPDF_CopyViewerPreferences(FPDF_DOCUMENT dest_doc, FPDF_DOCUMENT src_doc);
+
+#ifdef __cplusplus
+};
 #endif
 
+#endif //_FPDFPPO_H_
+