patch 8.1.0021: clang warns for undefined behavior

Problem:    Clang warns for undefined behavior.
Solution:   Move #ifdef outside of sprintf() call.(suggestion by Michael
            Jarvis, closes #2956)
diff --git a/src/version.c b/src/version.c
index 616a169..3208eca 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    21,
+/**/
     20,
 /**/
     19,