gnrc_ipv6_nib: always configure 802.15.4 long address

Our `gnrc_minimal` example configures the link-local address from the
IEEE 802.15.4 short address since it does not include 6Lo-ND.
This causes the application to be incompatible with our other GNRC
application that do include 6Lo-ND, since it [assumes][1] the link-local
address to be based on the EUI-64 for address resolution.

This enforces long addresses (aka EUI-64) for all IEEE 802.15.4 devices
when IPv6 is compiled in so `gnrc_minimal` is compatible again to the
rest.

Fixes #9910

[1]: https://tools.ietf.org/html/rfc6775#section-5.2