PM: cpuidle: Fix cpuidle_poll_state_init() prototype

Commit f8594220 (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: f8594220 (x86: PM: Make APM idle driver initialize polling state)
Cc: 4.14+ <stable@vger.kernel.org> # 4.14+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>