patch 8.1.2101: write_session_file() often defined but not used

Problem:    write_session_file() often defined but not used.
Solution:   Adjust the #ifdef. (Yegappan Lakshmanan, closes #4998)
diff --git a/src/version.c b/src/version.c
index cca023d..8feb2cc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -754,6 +754,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2101,
+/**/
     2100,
 /**/
     2099,