headers: Add missing __cdecl for fopen64() function

Signed-off-by: Martin Storsjö <martin@martin.st>
diff --git a/mingw-w64-headers/crt/stdio.h b/mingw-w64-headers/crt/stdio.h
index 89ce689..1cd6edc 100644
--- a/mingw-w64-headers/crt/stdio.h
+++ b/mingw-w64-headers/crt/stdio.h
@@ -569,7 +569,7 @@
 #endif
   _CRTIMP int __cdecl _flushall(void);
   FILE *__cdecl fopen(const char * __restrict__ _Filename,const char * __restrict__ _Mode) __MINGW_ATTRIB_DEPRECATED_SEC_WARN;
-  FILE *fopen64(const char * __restrict__ filename,const char * __restrict__  mode);
+  FILE *__cdecl fopen64(const char * __restrict__ filename,const char * __restrict__  mode);
   int __cdecl fputc(int _Ch,FILE *_File);
   _CRTIMP int __cdecl _fputchar(int _Ch);
   int __cdecl fputs(const char * __restrict__ _Str,FILE * __restrict__ _File);