tkutil.c: use rb_sym2str
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50309 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
tkutil.c: use rb_sym2str
not rb_sym_to_s, and fallback to rb_id2str and SYM2ID.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50309 b2dd03c8-39d4-4d8f-98ff-823fe69b080e