logging: cleanup in CMakeLists.txt

log_output.c was added in two places: unconditionally when logging
is enabled and when CONFIG_LOG_OUTPUT is set. Cleaning up to add
source file only when module is enabled.

Original commit was reverted. Now it is reapplied.
Original commit:af31cad8

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