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.
(cherry picked from commit 711f9e1)

Co-authored-by: Minmin Gong <gongminmin@msn.com>

Author: miss-islington

Misc/NEWS.d/next/Windows/2020-05-19-04-11-12.bpo-40677.qQbLW8.rst

@@ -0,0 +1 @@
+Manually define IO_REPARSE_TAG_APPEXECLINK in case some old Windows SDK doesn't have it.

Modules/_stat.c

@@ -40,6 +40,10 @@ typedef unsigned short mode_t;
 #  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 */