drivers/timer native_posix: Allow without BOARD_NATIVE_POSIX

This driver works both with native_posix and native_sim.
native_sim will eventually not set NATIVE_SIM_NATIVE_POSIX_COMPAT
(i.e. not pretend to be native_posix) so let's correct the
dependencies.

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