fix broken source link in nagios-nsca

source link is broken:
```
$ wget --spider http://dl.sf.net/nagios/nsca-2.7.2.tar.gz
Spider mode enabled. Check if remote file exists.
--2013-05-30 17:16:59--  http://dl.sf.net/nagios/nsca-2.7.2.tar.gz
Resolving dl.sf.net (dl.sf.net)... 216.34.181.138
Connecting to dl.sf.net (dl.sf.net)|216.34.181.138|:80... connected.
HTTP request sent, awaiting response... 302 Found
Location: http://sourceforge.net/downloads/nagios/nsca-2.7.2.tar.gz [following]
Spider mode enabled. Check if remote file exists.
--2013-05-30 17:16:59--  http://sourceforge.net/downloads/nagios/nsca-2.7.2.tar.gz
Resolving sourceforge.net (sourceforge.net)... 216.34.181.60
Connecting to sourceforge.net (sourceforge.net)|216.34.181.60|:80... connected.
HTTP request sent, awaiting response... 301 Moved Permanently
Location: http://sourceforge.net/projects/nagios/files/nsca-2.7.2.tar [following]
Spider mode enabled. Check if remote file exists.
--2013-05-30 17:17:00--  http://sourceforge.net/projects/nagios/files/nsca-2.7.2.tar
Connecting to sourceforge.net (sourceforge.net)|216.34.181.60|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
Remote file does not exist -- broken link!!!

```

For some mysterious reason the `.gz` extension is dropped during redirections, this patch replaces the source URL with a valid one.