dante: disable sched_getscheduler() - not implemented in musl

dante: disable sched_getscheduler() - not implemented in musl

musl doesn't come with an valid implementation of `sched_getscheduler()`;
it simply returns -ENOSYS for it. Without this option (and compile dante
with `sched_getscheduler()` enabled), you will get

    error: serverinit(): sched_getscheduler(2): failed to retrieve current
    cpuscheduling policy: Function not implemented

and dante won't start at all.

Ref: http://lists.alpinelinux.org/alpine-devel/3932.html
Ref: http://lists.alpinelinux.org/alpine-devel/3936.html
Signed-off-by: David Yang <mmyangfl@gmail.com>
[slightly reword commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(backported from aaf46a8fe23eca959164c1681ab3a37c6e746b05)