net: if: Add helper to return the first IPv4 address for iface

This is helper is only needed in socket multicast interface selection
where we need to get one address from the interface so that it will
tell (when getsockopt() is used), the interface IPv4 address where
multicast packets will be sent. This is private function which is not
needed in public headers so place the prototype to net_private.h file.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>