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.
diff --git a/src/version.c b/src/version.c
index 7bcb8c8..1829fa3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -768,6 +768,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1320,
+/**/
     1319,
 /**/
     1318,