bpo-40677: Define IO_REPARSE_TAG_APPEXECLINK explicitly (GH-20206)

This allows building with older versions of the Windows SDK where the value is not defined.
diff --git a/Modules/_stat.c b/Modules/_stat.c
index c7090c0..546e6a5 100644
--- a/Modules/_stat.c
+++ b/Modules/_stat.c
@@ -40,6 +40,10 @@
 #  define FILE_ATTRIBUTE_NO_SCRUB_DATA 0x20000
 #endif
 
+#ifndef IO_REPARSE_TAG_APPEXECLINK
+#  define IO_REPARSE_TAG_APPEXECLINK 0x8000001BL
+#endif
+
 #endif /* MS_WINDOWS */
 
 /* From Python's stat.py */