Make (and verify) public/ files compile under C.

public/fpdf_fwlevent.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 _FWL_EVENT_H
 #define _FWL_EVENT_H
 
 #include "fpdfview.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef int                     FPDF_INT32;
 typedef unsigned int FPDF_UINT32;
 typedef float           FPDF_FLOAT;
 //event type
 typedef enum
 {
         FWL_EVENTTYPE_Mouse = 0 ,
         FWL_EVENTTYPE_MouseWheel,       
         FWL_EVENTTYPE_Key               ,
 } FWL_EVENTTYPE;
@@ skipping 252 matching lines @@
         //FWL_EVENTTYPE.
     FPDF_UINT32 type;
         union
         {
                 struct FWL_EVENT_MOUSE mouse;
                 struct FWL_EVENT_MOUSEWHEEL wheel;
                 struct FWL_EVENT_KEY key;
         }s;
 };  
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif //_FWL_EVENT_H