common/sched: Fix ARM build following c/s 340edc3902

common/sched: Fix ARM build following c/s 340edc3902

The OSSTest smoke tests reports:

  sched_credit2.c: In function 'csched2_alloc_domdata':
  sched_credit2.c:3015:9: error: implicit declaration of function 'ERR_PTR' [-Werror=implicit-function-declaration]
           return ERR_PTR(-ENOMEM);
           ^
  sched_credit2.c:3015:9: error: nested extern declaration of 'ERR_PTR' [-Werror=nested-externs]

As the ERR infrastructure is part of the main scheduler interface now, include it from xen/sched-if.h

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>