fix the type of value that is returned by bit-shift expression.

src/hash.c

@@ -15,7 +15,7 @@
 static inline khint_t
 mrb_hash_ht_hash_func(mrb_state *mrb, mrb_value key)
 {
-  khint_t h = mrb_type(key) << 24;
+  khint_t h = (khint_t)mrb_type(key) << 24;
   mrb_value h2;
 
   h2 = mrb_funcall(mrb, key, "hash", 0, 0);