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.

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    577,
 /**/
     576,
 /**/