Bug 17116: Fix CSRF in import_borrowers.pl

Bug 17116: Fix CSRF in import_borrowers.pl

If an attacker can get an authenticated Koha user to visit their page
with the url below, they can change patrons' information

The exploit can be simulated triggering
  /tools/import_borrowers.pl?uploadborrowers=42

In that case it won't do anything wrong, but it you POST a valid file,
it could.

Test plan:
Trigger the url above
=> Without this patch, you will the result page
=> With this patch, you will get the "Wrong CSRF token" error.

Regression test:
Import a valid file from the import patron form, everything should go
fine.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>