MIPS: Include linux/cpu.h for arch_cpu_idle()

arch/mips/kernel/idle.c provides our implementation of the
arch_cpu_idle() function, but doesn't include the linux/cpu.h header
which declares it. This leads to a warning from sparse:

  arch/mips/kernel/idle.c:256:6: warning: symbol 'arch_cpu_idle' was not
    declared. Should it be static?

Fix this by including linux/cpu.h to get the declaration of
arch_cpu_idle().
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: trivial@kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17169/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/idle.c

@@ -11,6 +11,7 @@
  * as published by the Free Software Foundation; either version
  * 2 of the License, or (at your option) any later version.
  */
+#include <linux/cpu.h>
 #include <linux/export.h>
 #include <linux/init.h>
 #include <linux/irqflags.h>