patch 8.2.2899: Appveyor script does not detect nmake failure

Problem:    Appveyor script does not detect nmake failure.
Solution:   Explicitly check for executable. (Ken Takata, closes #8281)
diff --git a/src/version.c b/src/version.c
index 4b1a7ae..9e3ba0f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2899,
+/**/
     2898,
 /**/
     2897,