Bug 17600: Fix imports for svc/letters/preview

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

svc/letters/preview

@@ -19,9 +19,9 @@
 
 use Modern::Perl;
 use CGI qw( -utf8 );
-use C4::Auth;
+use C4::Auth qw( get_template_and_user );
 use C4::Context;
-use C4::Output;
+use C4::Output qw( output_html_with_http_headers );
 use C4::Circulation;
 use C4::Letters;
 use Koha::Checkouts;