samples posix uname: Fix sample yaml filtering

The sample was at the same time filtered for the whole
posix architecture and had native_posix as its integration platform.
This is selfcontradictory.

In any case, this sample cannot run in native_posix but
can run in native_sim.
So let's fix the filtering.

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