Bug 7701: Error when sending serial alerts

Bug 7701: Error when sending serial alerts

Probably due to bug 6875, C4::Letters is no longer used by C4::Serials, so that
when SendAlerts is called (exactly when this happens is a mystery to me), you
will get an error. Adding a "require C4::Letters;" and prefixing SendAlerts to
C4::Letters:: should fix it.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Patch fixes the perl error. Steps to reproduce:

1) Create a subscription and set patron notification to "Routling list" -
this is necessary, so that a link to email notifications shows up in the OPAC.
2) Search for the serial record in the OPAC and click on the more details
view from the subscriptions tab. Subscribe to email notifications.
3) Go back into the serials module and receive and issue for the serial.
Before the patch a nasty perl error will be shown, after the patch
receiving works.

I couldn't confirm that the email was sent out with my setup, but this patch
certainly fixes serial receive.

[Update 2011-02-29] Fixed the typo and retested. Looks good, but couldn't check
if the mail was actually sent.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>