Fix annotation for xdg_mime_list_parents

There is a fatal crash in Sugar when retrieving mimetypes for some Journal
entries when contructing the icon palette (for both the Journal list view
and detail view). We isolated the problem to xdgmime.c, the call to
xdg_get_mime_parents. Dnarvaez observed that the annotation for the
return argument was missing and that the annotation for the return for
mime_list_parents was wrong. This patch fixes these annotations.

Fixes #4530