Guard WIN32_LEAN_AND_MEAN in the C++20 module

Avoid a macro redefinition warning when WIN32_LEAN_AND_MEAN is
already provided through compiler flags, for example with
-DWIN32_LEAN_AND_MEAN.

This follows the approach previously adopted in #1729.

src/fmt.cc

@@ -101,7 +101,9 @@ module;
 #    include <ext/stdio_filebuf.h>
 #    include <ext/stdio_sync_filebuf.h>
 #  endif
-#  define WIN32_LEAN_AND_MEAN
+#  ifndef WIN32_LEAN_AND_MEAN
+#    define WIN32_LEAN_AND_MEAN
+#  endif
 #  include <windows.h>
 #endif