Bug 21898: Add basket for TT syntax

Without this patch the basket's info won't be available for the TT
syntax.

With this patch you can now use [% basket.basketname %]

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

C4/Letters.pm

@@ -1526,6 +1526,12 @@ sub _get_tt_params {
             plural   => 'article_requests',
             pk       => 'id',
         },
+        aqbasket => {
+            module   => 'Koha::Acquisition::Baskets',
+            singular => 'basket',
+            plural   => 'baskets',
+            pk       => 'basketno',
+        },
         biblio => {
             module   => 'Koha::Biblios',
             singular => 'biblio',