Kill following warning.

../../../ext/psych/yaml/emitter.c: In function ‘yaml_emitter_write_block_scalar_hints’:
../../../ext/psych/yaml/emitter.c:2196:20: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
         chomp_hint = "-";
                    ^

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60268 b2dd03c8-39d4-4d8f-98ff-823fe69b080e