logging: Add mising handling of function prefix

When printk is redirected to the logging it is using level 0.
Level 0 was missing a define used for detection of function
prefix use which lead to compilation failure when logging v2
was used with CONFIG_LOG_PRINTK=y.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>