ARM: shmobile: r8a7779: hide unused r8a7779_platform_cpu_kill

After the cleanup in r8a7779_smp_prepare_cpus(), the only remaining caller of
r8a7779_platform_cpu_kill() is in an ifdef, which leads to a build warning
without CONFIG_HOTPLUG_CPU:

arch/arm/mach-shmobile/smp-r8a7779.c:26:12: error: 'r8a7779_platform_cpu_kill' defined but not used [-Werror=unused-function]

This moves the function inside of that #ifdef to avoid the warning.

Fixes: 62f55ce6 ("ARM: shmobile: r8a7779: Stop powering down secondary CPUs during early boot")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>