Bug 20276: Fix GetCourseItem call to get itemnumber

Bug 20276: Fix GetCourseItem call to get itemnumber

In 17.05.x :
In course_reserves/add_items.pl we now get the item via Objects.
This line:
 66     my $course_item = GetCourseItem( itemnumber => $item->{'itemnumber'} );
Should be:
 66     my $course_item = GetCourseItem( itemnumber => $item->itemnumber );

This is corrected since 17.11 by :
Bug 18276: Remove GetBiblioFromItemNumber - Course reserves

Test plan :
1) Create a course reserve and add items.
2) Correct information must be displayed on the detail page of the course
reserve, on staff and OPAC interface.
3) Try to add a non existing barcode, you get the page with a message

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>