Undefined symbol make_unix_socket

Undefined symbol make_unix_socket

syslog_direct_create defined in (lib/syslog-direct.c) uses make_unix_socket
which is currently undefined on the windows build.

We either can remove the new file from the chain but this patch proposes
to define a wrapper to make_unix_socket in which we return EINVAL.

This will avoid this kind of problems in the future.

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>