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/optionstr.c b/src/optionstr.c
index d667e53..e82654a 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -3565,7 +3565,7 @@
  * Process the new 'tabpanelopt' option value.
  */
     char *
-did_set_tabpanelopt(optset_T *args)
+did_set_tabpanelopt(optset_T *args UNUSED)
 {
     if (tabpanelopt_changed() == FAIL)
 	return e_invalid_argument;