samples: rust: select AUXILIARY_BUS instead of depending on it

CONFIG_AUXILIARY_BUS cannot be enabled explicitly, and unless we select
it we have no way to include it (and thus to enable the auxiliary driver
sample) unless a driver happens to do it for us.

Fixes: 96609a19 ("samples: rust: add Rust auxiliary driver sample")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250515-aux_bus-v2-1-47c70f96ae9b@nvidia.com


Signed-off-by: Danilo Krummrich <dakr@kernel.org>