package/readline: add host variant

Needed by SoftEther VPN.

host-readline will also be needed by host-pcre, which in turn will
be needed by the yet-to-be-released leafnode2 package.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>