patch 8.1.1437: code to handle callbacks is duplicated

Problem:    Code to handle callbacks is duplicated.
Solution:   Add callback_T and functions to deal with it.
diff --git a/src/version.c b/src/version.c
index 352969f..c7c19f3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -768,6 +768,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1437,
+/**/
     1436,
 /**/
     1435,