Bug 29435: include missing subroutine from C4::AuthoritiesMarc

The OPAC authority details page is broken when the 'AuthDisplayHierarchy'
System Preference is set to 'Enable',

This patch fixes that.

Test plan:
1) Set AuthDisplayHierarchy to 'Enable' and try to view the details
   page of an authority in the OPAC -- the page is broken.
2) Apply this patch.
3) Refresh the page; this time the authority details are displayed.

Signed-off-by: David Nind <david@davidnind.com>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

opac/opac-authoritiesdetail.pl

@@ -42,7 +42,7 @@ use C4::Auth qw( get_template_and_user );
 use C4::Biblio qw( GetMarcUrls );
 use C4::Context;
 use C4::Output qw( output_html_with_http_headers );
-use C4::AuthoritiesMarc qw( GetAuthority BuildSummary GetTagsLabels );
+use C4::AuthoritiesMarc qw( GetAuthority BuildSummary GetTagsLabels GenerateHierarchy );
 use CGI qw ( -utf8 );
 use C4::Koha;