sched/cpuacct/UML: Fix header file dependency bug on the UML build

The cpuacct split caused this build failure on UML:

  kernel/sched/cpuacct.c:94:2: error: implicit declaration of  function 'ERR_PTR'

Cc: Li Zefan <lizefan@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@kernel.org>

kernel/sched/cpuacct.c

@@ -6,6 +6,7 @@
 #include <linux/seq_file.h>
 #include <linux/rcupdate.h>
 #include <linux/kernel_stat.h>
+#include <linux/err.h>
 
 #include "sched.h"