[syslog-ng] IPv6 addresses logged as 127.0.0.1

When using the template parameter $SOURCEIP, connections originating from IPv6 addresses are logged as "127.0.0.1". The following patch fixes that issue. Listening on :: on dual-stack hosts will lead to IPv4 connections being logged in IPv6-mapped format, e.g. ::ffff:10.11.12.13. This is not what people usually expect, but it's not wrong. If desired, the place to fix that would be g_sockaddr_inet6_format().