Bluetooth: controller: split: include proper header file

After removal of legacy controller ll_adv_aux.h does not exist anymore.
This is fixed in this PR by including ull_adv_aux.h instead.
The error only shows up when Advertisement Extensions are enabled
in the LL (CONFIG_BT_CTLR_ADV_EXT=y)

Signed-off-by: Andries Kruithof <Andries.Kruithof@nordicsemi.no>