* st.c (st_keys): fix not to use Qundef in st.c.

* include/ruby/st.h: define modified prototype.

* hash.c (rb_hash_keys): use modified st_keys().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43894 b2dd03c8-39d4-4d8f-98ff-823fe69b080e