subsys logging: Enable native backend if possible

Let's always enable the native_posix backend even
if the serial is used.
Both can be used at the same time, and:
a) users expect the logger output in the invoking shell even if they
enable the UART
b) Since printk is routed to the logger twister actually needs it
for tests to pass as for native platforms it looks into the stdout
of the process.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>