Resolved warning C4996: 'fileno': The POSIX name for this item is deprecated. Instead, use the ISO C and C++ conformant name: _fileno. (#3930)

Author: matt77hias

Diff for: src/os.cc

@@ -188,6 +188,8 @@ int buffered_file::descriptor() const {
 #    undef fileno
 #  endif
   int fd = FMT_POSIX_CALL(fileno(file_));
+#elif defined(_WIN32)
+  int fd = _fileno(file_);
 #else
   int fd = fileno(file_);
 #endif