patch 8.2.0879: compiler warning for unused function argument

Problem:    Compiler warning for unused function argument.
Solution:   Add UNUSED.
diff --git a/src/search.c b/src/search.c
index 802718d..e0b0f9b 100644
--- a/src/search.c
+++ b/src/search.c
@@ -3170,7 +3170,7 @@
     searchstat_T	*stat,
     int			recompute,
     int			maxcount,
-    long		timeout)
+    long		timeout UNUSED)
 {
     int		    save_ws = p_ws;
     int		    wraparound = FALSE;