microblaze: Fix sparse warning because of missing cpu.h header

Warning:
arch/microblaze/kernel/process.c:164:6: warning: symbol 'arch_cpu_idle'
was not declared. Should it be static?
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/kernel/process.c

@@ -8,6 +8,7 @@
  * for more details.
  */
 
+#include <linux/cpu.h>
 #include <linux/export.h>
 #include <linux/sched.h>
 #include <linux/pm.h>