meson: Check for sched_get_priority_min()

meson: Check for sched_get_priority_min()

virProcessSetScheduler() uses not just sched_setscheduler() but
also sched_get_priority_{min,max}(). Currently we assume that
the former being available implies that the latter are as well,
but that's not the case for at least GNU/Hurd.

Make sure all functions are actually available before
attempting to use them.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>