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().