patch 8.1.0577: tabpage right-click menu never shows "Close tab"

Problem:    Tabpage right-click menu never shows "Close tab".
Solution:   Always create the "Close tab" item but ignore the event if there
            is only one tab.
diff --git a/src/version.c b/src/version.c
index 29fdfb2..0600217 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    577,
+/**/
     576,
 /**/
     575,