patch 8.2.1298: compiler warning for unused argument in small version

Problem:    Compiler warning for unused argument in small version.
Solution:   Add UNUSED.
diff --git a/src/scriptfile.c b/src/scriptfile.c
index ce9c15d..2a83208 100644
--- a/src/scriptfile.c
+++ b/src/scriptfile.c
@@ -114,7 +114,7 @@
  * "is_sfile" is TRUE for <sfile> itself.
  */
     char_u *
-estack_sfile(int is_sfile)
+estack_sfile(int is_sfile UNUSED)
 {
     estack_T	*entry;
 #ifdef FEAT_EVAL