tests: unit: cbprintf: fix for filtered prototypes

cbprintf should not pull in stdio.h unless it needs to, specifically
to get the FILE type for prototyping the stream substitutions.  The
unit test assumes that these functions are always declared.  For unit
testing where Kconfig isn't involved we need to tell the build system
to always provide them.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>