patch 8.1.1576: compiler warning for unused argument

Problem:    Compiler warning for unused argument.
Solution:   Add "UNUSED" annotation. (Dominique Pelle, closes #4570)
diff --git a/src/ui.c b/src/ui.c
index db878e6..896ab0c 100644
--- a/src/ui.c
+++ b/src/ui.c
@@ -1484,7 +1484,7 @@
  */
     static void
 clip_invert_rectangle(
-	Clipboard_T	*cbd,
+	Clipboard_T	*cbd UNUSED,
 	int		row_arg,
 	int		col_arg,
 	int		height_arg,