ARM: imx: cpuidle-imx5: Include "cpuidle.h" header file

The following build warning is seen with W=1:

warning: no previous prototype for ‘imx5_cpuidle_init’
[-Wmissing-prototypes] int __init imx5_cpuidle_init(void)

Fix this warning by including "cpuidle.h".

Signed-off-by: Marco Franchi <marco.franchi@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>