Bug 16859: Fix wrong item field name in export.pl

Bug 16859: Fix wrong item field name in export.pl

Field callnumber should be itemcallnumber.
For completeness, prefixing the joined fields with items table prefix.

Note: You should expect Tools/Export to export only the items within a
given itemcallnumber range, but if the biblio has one item in that
range, the biblionumber is selected for export with ALL items. The script
is designed that way (first select biblio numbers based on criteria, and
run export with those biblio numbers).

Test plan:
[1] Select a biblio N with one item with say itemcallnumber X.
[2] Goto Tools/Export. Select biblio range N,N and no itemcallnumber
    range. The biblio should be exported.
[3] Select biblio range N,N and itemcallnumber range A,B. No output.
[4] Select biblio range N,N and itemcallnumber range A,<empty>. Output.
[5] Select biblio range N,N and itemcallnumber range <empty>,X. Output.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>