reinject_email: fix connecting for ipv6-only hosts

reinject_email: fix connecting for ipv6-only hosts

When configuring PMG only with ipv6 addresses, reinject_email after processing
fails to connect to the postfix/smtpd instance (with EINVAL).

Setting the host to '::FFFF:127.0.0.1' fixes the issue.

Tested with:
* an ipv6only host (no ipv4 configured)
* a host with ipv6 disabled via sysctl:
```
net.ipv6.conf.all.disable_ipv6=1
net.ipv6.conf.default.disable_ipv6=1
net.ipv6.conf.lo.disable_ipv6=1
```
* a host with dual-stack setup

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>