soc: nxp: rw: Fix nxp_nbu_init implicit definition warning

nxp_nbu_init extern definition is under CONFIG_NXP_RW6XX_BOOT_HEADER
and this produces an implicite definition warning when
the Kconfig is disabled. This is the case when both BLE Kconfig
and MCUboot bootloader Kconfig are enabled.

Signed-off-by: Yassine El Aissaoui <yassine.elaissaoui@nxp.com>