patch 8.2.4101: warning for unused argument in tiny version

Problem:    Warning for unused argument in tiny version.
Solution:   Add "UNUSED".
diff --git a/src/getchar.c b/src/getchar.c
index 6434b4d..ec9cfae 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -3811,7 +3811,7 @@
 }
 
     int
-do_cmdkey_command(int key, int flags)
+do_cmdkey_command(int key UNUSED, int flags)
 {
     int	    res;
 #ifdef FEAT_EVAL