cpuidle: header file stubs must be "static inline"

An x86_64 allmodconfig build produces these errors:

x86_64-linux-gnu-ld: kernel/sched/core.o: in function `cpuidle_poll_time':
core.c:(.text+0x230): multiple definition of `cpuidle_poll_time'; arch/x86/=
kernel/process.o:process.c:(.text+0xc0): first defined here

(and more)

Fixes: 259231a0 ("cpuidle: add poll_limit_ns to cpuidle_device structure")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/linux/cpuidle.h

@@ -170,7 +170,7 @@ static inline int cpuidle_enter(struct cpuidle_driver *drv,
 				struct cpuidle_device *dev, int index)
 {return -ENODEV; }
 static inline void cpuidle_reflect(struct cpuidle_device *dev, int index) { }
-extern u64 cpuidle_poll_time(struct cpuidle_driver *drv,
+static inline u64 cpuidle_poll_time(struct cpuidle_driver *drv,
 			     struct cpuidle_device *dev)
 {return 0; }
 static inline int cpuidle_register_driver(struct cpuidle_driver *drv)