Fix scheduler support check

Fix scheduler support check

Commit 94cc577807ba tried fixing build on systems that did not have
SCHED_BATCH or SCHED_IDLE defined.  But instead of changing it to
conditional support, it rather completely disabled the support for
setting any scheduler.  Since then, such old systems are not
supported, but rather than reverting that commit, let's change that to
the conditional support.  That way any addition to the list of
schedulers can follow the same style so that we're consistent in the
future.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>