sched: Fix build warning without CONFIG_SYSCTL

IF CONFIG_SYSCTL is n, build warn:

kernel/sched/core.c:1782:12: warning: ‘sysctl_sched_uclamp_handler’ defined but not used [-Wunused-function]
 static int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~

sysctl_sched_uclamp_handler() is used while CONFIG_SYSCTL enabled,
wrap all related code with CONFIG_SYSCTL to fix this.

Fixes: 3267e015 ("sched: Move uclamp_util sysctls to core.c")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>