chrt: Use sched_setscheduler system call directly

chrt: Use sched_setscheduler system call directly

musl libc does not support the sched_setscheduler library function
because the underlying Linux system call does not confirm to Posix;
this patch makes chrt use the system call directly

[kzak@redhat.com:
    - note that musl libc implements sched_setscheduler()
      but returns -ENOSYS all time...
    - add ifdefs to the patch
    - make sure we include syscall.h]

References: http://git.musl-libc.org/cgit/musl/commit/src/sched/sched_setscheduler.c?id=1e21e78bf7a5c24c217446d8760be7b7188711c2
Addresses: https://github.com/karelzak/util-linux/issues/943
Signed-off-by: Karel Zak <kzak@redhat.com>