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>