Bug 8437: Exclude export.pl from plack

Bug 8437: Exclude export.pl from plack

When attempting to download large files from Koha plack can timeout

Excluding the script from plack is a simple fix until we have a more permanent fix for this
issue.

To test:
1 - Try to export your entire DB from Tools->Export
2 - If big enough, it fails
3 - Apply patch, copy changes to /etc/koha/apache-shared-intranet-plack.conf
4 - Restart all the things
5 - Repeat export, it succeeds

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>