sched-if.h: include xen/err.h

This is to address an Arm build failure after commit d86c9aeae6 ("xen:
sched/Credit2: fix bug when moving CPUs between two Credit2 cpupools").
The inclusion is done in xen/sched-if.h to match newer versions of the
code.

Signed-off-by: Jan Beulich <jbeulich@suse.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;