Fix bug in parser for simpleselp that prevented bare subselps.

In the grammar, the production is:

simple_selector
  : element_name [ HASH | class | attrib | pseudo ]*
  | [ HASH | class | attrib | pseudo ]+
  ;

Whereas the current code implemented only:

simple_selector
  : element_name [ HASH | class | attrib | pseudo ]*
  ;