patch 8.1.1320: it is not possible to track changes to a buffer

Problem:    It is not possible to track changes to a buffer.
Solution:   Add listener_add() and listener_remove(). No docs or tests yet.

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1320,
 /**/
     1319,
 /**/