reboot: Fix build warning without CONFIG_SYSCTL

If CONFIG_SYSCTL is n, build warn:

kernel/reboot.c:443:20: error: ‘kernel_reboot_sysctls_init’ defined but not used [-Werror=unused-function]
 static void __init kernel_reboot_sysctls_init(void)
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~

Move kernel_reboot_sysctls_init() to #ifdef block to fix this.

Fixes: 06d17766 ("kernel/reboot: move reboot sysctls to its own file")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>