Fix -Wunused-function warnings on Windows.

core/src/fpdfapi/fpdf_font/fpdf_font_cid.cpp

Index: core/src/fpdfapi/fpdf_font/fpdf_font_cid.cpp
diff --git a/core/src/fpdfapi/fpdf_font/fpdf_font_cid.cpp b/core/src/fpdfapi/fpdf_font/fpdf_font_cid.cpp
index 6c728242e6d1a39e5430eaf44a3d87930219fa95..5927511ebf71cd5b5b54fc3730c8f5d939409186 100644
--- a/core/src/fpdfapi/fpdf_font/fpdf_font_cid.cpp
+++ b/core/src/fpdfapi/fpdf_font/fpdf_font_cid.cpp
@@ -832,6 +832,8 @@ FX_WORD CPDF_CIDFont::CIDFromCharCode(FX_DWORD charcode) const {
 FX_BOOL CPDF_CIDFont::IsVertWriting() const {
   return m_pCMap ? m_pCMap->IsVertWriting() : FALSE;
 }
+
+#if _FXM_PLATFORM_ != _FXM_PLATFORM_WINDOWS_
 extern FX_DWORD FPDFAPI_CharCodeFromCID(const FXCMAP_CMap* pMap, FX_WORD cid);
 static FX_DWORD _EmbeddedCharcodeFromUnicode(const FXCMAP_CMap* pEmbedMap,
                                              int charset,
@@ -857,6 +859,8 @@ static FX_DWORD _EmbeddedCharcodeFromUnicode(const FXCMAP_CMap* pEmbedMap,
   }
   return 0;
 }
+#endif  // _FXM_PLATFORM_ != _FXM_PLATFORM_WINDOWS_
+
 static FX_WCHAR _EmbeddedUnicodeFromCharcode(const FXCMAP_CMap* pEmbedMap,
                                              int charset,
                                              FX_DWORD charcode) {