Currently, dummy_mark variable makes following warnings. Let's fix it.

../../../ext/psych/yaml/parser.c: In function ‘yaml_parser_parse_block_sequence_entry’:
../../../ext/psych/yaml/parser.c:762:21: warning: variable ‘dummy_mark’ set but not used [-Wunused-but-set-variable]
         yaml_mark_t dummy_mark;     /* Used to eliminate a compiler warning. */
                     ^

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60263 b2dd03c8-39d4-4d8f-98ff-823fe69b080e