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>

xen/include/xen/sched-if.h

@@ -9,6 +9,7 @@
 #define __XEN_SCHED_IF_H__
 
 #include <xen/percpu.h>
+#include <xen/err.h>
 
 /* A global pointer to the initial cpupool (POOL0). */
 extern struct cpupool *cpupool0;