Fix -Wnon-virtual-dtor compiler warnings.

fpdfsdk/src/fpdf_sysfontinfo.cpp

 // 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
 
 #include "../include/fsdk_define.h"
 #include "../include/fpdf_sysfontinfo.h"
 
 class CSysFontInfo_Ext FX_FINAL : public IFX_SystemFontInfo
@@ skipping 57 matching lines @@
                         return TRUE;
                 }
                 return FALSE;
         }
 
         virtual void            DeleteFont(void* hFont)  FX_OVERRIDE
         {
                 if (m_pInfo->DeleteFont)
                         m_pInfo->DeleteFont(m_pInfo, hFont);
         }
+
+private:
+        ~CSysFontInfo_Ext() { }
 };
 
 DLLEXPORT void STDCALL FPDF_AddInstalledFont(void* mapper, const char* name, int charset)
 {
         ((CFX_FontMapper*)mapper)->AddInstalledFont(name, charset);
 }
 
 DLLEXPORT void STDCALL FPDF_SetSystemFontInfo(FPDF_SYSFONTINFO* pFontInfoExt)
 {
         if (pFontInfoExt->version != 1) return;
@@ skipping 66 matching lines @@
         pFontInfoExt->GetFaceName = DefaultGetFaceName;
         pFontInfoExt->GetFont = DefaultGetFont;
         pFontInfoExt->GetFontCharset = DefaultGetFontCharset;
         pFontInfoExt->GetFontData = DefaultGetFontData;
         pFontInfoExt->MapFont = DefaultMapFont;
         pFontInfoExt->Release = DefaultRelease;
         pFontInfoExt->version = 1;
         pFontInfoExt->m_pFontInfo = pFontInfo;
         return pFontInfoExt;
 }