PM: cpuidle: Fix cpuidle_poll_state_init() prototype

PM: cpuidle: Fix cpuidle_poll_state_init() prototype

commit d7212cfb05ba802bea4dd6c90d61cfe6366ea224 upstream.

Commit f85942207516 (x86: PM: Make APM idle driver initialize polling
state) made apm_init() call cpuidle_poll_state_init(), but that only
is defined for CONFIG_CPU_IDLE set, so make the empty stub of it
available for CONFIG_CPU_IDLE unset too to fix the resulting build
issue.

Fixes: f85942207516 (x86: PM: Make APM idle driver initialize polling state)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>