patch 8.1.2188: build error for missing define

Problem:    Build error for missing define.
Solution:   Add missing change.
diff --git a/src/version.c b/src/version.c
index c55af5a..f11a020 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2188,
+/**/
     2187,
 /**/
     2186,