BibFormat: fix external function for libxslt
BibFormat: fix external function for libxslt

* Fixes an issue causing the external XSLT function
  eval_bibformat to break, by changing the import of
  format_with_format_template() inside eval_bibformat_libxslt()
  to account for recent changes to BibFormat.

tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio