net: lib: lwm2m: Add missing function declarations
net: lib: lwm2m: Add missing function declarations

Get-function declarations for fw update state and result were
missing from the engine header.

Signed-off-by: Jarno Lamsa <e7db9160c1525e70833a92708551a8058cdb9643@nordicsemi.no>

zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr