patch 8.1.1735: can't build with tiny features

Problem:    Can't build with tiny features.
Solution:   Add missing #ifdefs.
diff --git a/src/json.c b/src/json.c
index c9a6230..2a5b5e4 100644
--- a/src/json.c
+++ b/src/json.c
@@ -1126,7 +1126,6 @@
     reader->js_used = used_save;
     return ret;
 }
-#endif
 
 /*
  * "js_decode()" function
@@ -1176,3 +1175,4 @@
     rettv->v_type = VAR_STRING;
     rettv->vval.v_string = json_encode(&argvars[0], 0);
 }
+#endif