clearn up warning in hash.c

/home/fleuria/code/mruby/src/hash.c:159:1: warning: ‘mrb_hash_dp`defined but not used [-Wunused-function]
 mrb_hash_dup(mrb_state *mrb, mrb_value hash)
     ^

this commit defines Hash#dup to take advantage of mrb_hash_dup, however
it seems that Hash#dup is not in ISO standard.

src/hash.c

@@ -987,6 +987,7 @@ mrb_init_hash(mrb_state *mrb)
   mrb_define_method(mrb, h, "member?",         mrb_hash_has_key,     MRB_ARGS_REQ(1)); /* 15.2.13.4.21 */
   mrb_define_method(mrb, h, "replace",         mrb_hash_replace,     MRB_ARGS_REQ(1)); /* 15.2.13.4.23 */
   mrb_define_method(mrb, h, "shift",           mrb_hash_shift,       MRB_ARGS_NONE()); /* 15.2.13.4.24 */
+  mrb_define_method(mrb, h, "dup",             mrb_hash_dup,         MRB_ARGS_NONE());
   mrb_define_method(mrb, h, "size",            mrb_hash_size_m,      MRB_ARGS_NONE()); /* 15.2.13.4.25 */
   mrb_define_method(mrb, h, "store",           mrb_hash_aset,        MRB_ARGS_REQ(2)); /* 15.2.13.4.26 */
   mrb_define_method(mrb, h, "value?",          mrb_hash_has_value,   MRB_ARGS_REQ(1)); /* 15.2.13.4.27 */