lib: os: cbprintf: Suppress sizeof-array-decay warning

Z_CBPRINTF_ARG_SIZE macro is called for each argument in
logging macros. If argument is a string literal an intention
of this macro is to return size of a pointer. Suppressing
warning which appears in that case.

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