Bug 31518: Fix syntax error on request.tt that prevents showing hidden item count

Bug 31518: Fix syntax error on request.tt that prevents showing hidden item count

The variable set by request.pl doesn't have biblio prefix, it is just
hiddencount.

To test:
 1) Log in with a staff member account that has "Lost items in staff
 interface" setting set to "Hidden by default" in the patron category
 settings
 2) Add a lost item to biblio
 3) Go place a hold on staff interface (request.pl) notice the broken
 link on the request page: "Show all items ( hidden)"
 4) After applying the patch it should read "Show all items (1 hidden)"

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>