Fix cannot capture title of literal_block node by xref

--HG--
extra : amend_source : db5b1330ea9aa1650c9f775f458d3d3253cdc485
extra : transplant_source : %97xeB%0D%19%B0%20%EC%D8V%10rC%22B%07Ywi
Merged in tk0miya/sphinx (pull request #291)

Fix cannot capture title of literal_block node by xref