patch 9.1.1457: compile warning with tabpanelopt

Problem:  compile warning with tabpanelopt
Solution: declare arg as UNUSED (John Marriott)

closes: #17528

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/option.c b/src/option.c
index 85c3705..150e562 100644
--- a/src/option.c
+++ b/src/option.c
@@ -8847,7 +8847,7 @@
  * Process the new 'showtabpanel' option value.
  */
     char *
-did_set_showtabpanel(optset_T *args)
+did_set_showtabpanel(optset_T *args UNUSED)
 {
     shell_new_columns();
     return NULL;