nimble/ll: Fix build error when ADV EXT is disabled
nimble/ll: Fix build error when ADV EXT is disabled

This fixes following build error:

ble_ll_adv.c:2054: undefined reference to 'ble_ll_hci_adv_mode_ext'

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble