net: ip: build IEEE 802.15.4 L2 without IP support

While a previous change had already decoupled the IEEE 802.15.4 L2 from
IP upper layers, this only worked when at least one other interface of
the device supported IP.

This change removes this requirement and thereby fixes a build error
that occurred when disabling IP support while maintaining IEEE 802.15.4
L2 support.

Fixes: #48718

Signed-off-by: Florian Grandel <jerico.dev@gmail.com>