Bug 20851: Add module to circ/article-request-slip.pl

Bug 20851: Add module to circ/article-request-slip.pl

See bug 20468, comment7.
Although the script seems to compile well, we better add the module here.
Trivial change.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>