libtirpc: Support ipv6 in DISTRO_FEATURES

If the ipv6 feature for the distribution is not set, the package should not
contain settings for ipv6. This makes rpcbind doesn't try to bind to a IPv6
socket, and complain that this fails.

Signed-off-by: Jörg Sommer <joerg.sommer@navimatix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>