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/gui_gtk_x11.c b/src/gui_gtk_x11.c
index dbddb0f..898e2af 100644
--- a/src/gui_gtk_x11.c
+++ b/src/gui_gtk_x11.c
@@ -3337,9 +3337,7 @@
     GtkWidget *menu;
 
     menu = gtk_menu_new();
-    if (first_tabpage->tp_next != NULL)
-	add_tabline_menu_item(menu, (char_u *)_("Close tab"),
-							  TABLINE_MENU_CLOSE);
+    add_tabline_menu_item(menu, (char_u *)_("Close tab"), TABLINE_MENU_CLOSE);
     add_tabline_menu_item(menu, (char_u *)_("New tab"), TABLINE_MENU_NEW);
     add_tabline_menu_item(menu, (char_u *)_("Open Tab..."), TABLINE_MENU_OPEN);