variable.c: use size_t to avoid integer type mismatch.

src/variable.c

@@ -486,7 +486,7 @@ mrb_value
 mrb_obj_iv_inspect(mrb_state *mrb, struct RObject *obj)
 {
   iv_tbl *t = obj->iv;
-  int len = iv_size(mrb, t);
+  size_t len = iv_size(mrb, t);
 
   if (len > 0) {
     const char *cn = mrb_obj_classname(mrb, mrb_obj_value(obj));