Bug 26331: Make svc/letters/preview executable

Bug 26331: Make svc/letters/preview executable

Without this patch, you can't preview letters when running Koha in CGI mode.

To test:
1. Run Koha as CGI (and not Plack)
2. Go to /cgi-bin/koha/tools/letter.pl?op=add_form&branchcode=&module=circulation&code=CHECKIN
3. Try to preview the notice (using a valid barcode)
4. Note in the browser console that svc/letters/preview is generating a 500 error

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>