patch 8.1.1138: plugins don't get notified when the popup menu changes

Problem:    Plugins don't get notified when the popup menu changes.
Solution:   Add the CompleteChanged event. (Andy Massimino. closes #4176)
diff --git a/src/proto/autocmd.pro b/src/proto/autocmd.pro
index 8c6d18f..a448dad 100644
--- a/src/proto/autocmd.pro
+++ b/src/proto/autocmd.pro
@@ -26,6 +26,7 @@
 int has_cmdundefined(void);
 int has_funcundefined(void);
 int has_textyankpost(void);
+int has_completechanged(void);
 void block_autocmds(void);
 void unblock_autocmds(void);
 int is_autocmd_blocked(void);