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.

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1437,
 /**/
     1436,
 /**/