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>

include/linux/cpuidle.h

@@ -225,7 +225,7 @@ static inline void cpuidle_coupled_parallel_barrier(struct cpuidle_device *dev,
 }
 #endif
 
-#ifdef CONFIG_ARCH_HAS_CPU_RELAX
+#if defined(CONFIG_CPU_IDLE) && defined(CONFIG_ARCH_HAS_CPU_RELAX)
 void cpuidle_poll_state_init(struct cpuidle_driver *drv);
 #else
 static inline void cpuidle_poll_state_init(struct cpuidle_driver *drv) {}