patch 8.1.1568: strftime() test fails on MS-Windows

Problem:    Strftime() test fails on MS-Windows.
Solution:   Skip the check for using the $TZ environment variable.
diff --git a/src/version.c b/src/version.c
index 677279b..68704cc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1568,
+/**/
     1567,
 /**/
     1566,