Update CacheFile.cc (#916)

On Windows, ftell returns a 32 bits signed integer.
On Windows we had to use _ftelli64 to return a signed 64 bits integer.

lib/src/CacheFile.cc

@@ -66,7 +66,11 @@ void CacheFile::append(const char *data, size_t length)
 size_t CacheFile::length()
 {
     if (file_)
+#ifdef _WIN32
+        return _ftelli64(file_);
+#else
         return ftell(file_);
+#endif
     return 0;
 }