patch 8.2.1456: MS-Windows: test files are not deleted

Problem:    MS-Windows: test files are not deleted.
Solution:   use "del" instead of $(DEL).
diff --git a/src/version.c b/src/version.c
index e873a9c..cc3b9fd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1456,
+/**/
     1455,
 /**/
     1454,