Codechange: Compile fmt internals in a separate translation unit (#13527)

Instead of defining FMT_HEADER_ONLY to include fmt internals
in the headers included by all fmt-using translation units.

src/os/windows/win32.cpp

@@ -20,6 +20,7 @@
 #include <shlobj.h> /* SHGetFolderPath */
 #include <shellapi.h>
 #include <winnls.h>
+#include <io.h>
 #include "win32.h"
 #include "../../fios.h"
 #include "../../core/alloc_func.hpp"