Bluetooth: Controller: lll: Fix warning when building for Nordic

[140/235] Building C object zephyr/subsys/bluetooth/controller/
    CMakeFiles/subsys__bluetooth__controller.dir/ll_sw/ull_sched.c.obj
In file included from zephyr/subsys/bluetooth/controller/ll_sw/
    ull_sched.c:28:
zephyr/subsys/bluetooth/controller/ll_sw/nordic/lll/lll_adv_pdu.h:
     In function 'lll_adv_pdu_linked_next_get':
zephyr/subsys/bluetooth/controller/ll_sw/nordic/lll/lll_adv_pdu.h:8:32:
     warning: implicit declaration of function 'MROUND'
     [-Wimplicit-function-declaration]
    8 | #define PDU_ADV_MEM_SIZE       MROUND(PDU_AC_LL_HEADER_SIZE + \
      |                                ^~~~~~
zephyr/subsys/bluetooth/controller/ll_sw/nordic/lll/lll_adv_pdu.h:12:11:
     note: in expansion of macro 'PDU_ADV_MEM_SIZE'
   12 |           PDU_ADV_MEM_SIZE - \
      |           ^~~~~~~~~~~~~~~~
zephyr/subsys/bluetooth/controller/ll_sw/nordic/lll/lll_adv_pdu.h:197:9:
     note: in expansion of macro 'PDU_ADV_NEXT_PTR'
  197 |  return PDU_ADV_NEXT_PTR(pdu);

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>