sched/rt: fix build error when CONFIG_SYSCTL is disable

Avoid random build errors which do not select
CONFIG_SYSCTL by depending on it in Kconfig.

This fixes the following warning:

In file included from kernel/sched/build_policy.c:43:
At top level:
kernel/sched/rt.c:3017:12: error: ‘sched_rr_handler’ defined but not used [-Werror=unused-function]
 3017 | static int sched_rr_handler(struct ctl_table *table, int write, void *buffer,
      |            ^~~~~~~~~~~~~~~~
kernel/sched/rt.c:2978:12: error: ‘sched_rt_handler’ defined but not used [-Werror=unused-function]
 2978 | static int sched_rt_handler(struct ctl_table *table, int write, void *buffer,
      |            ^~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [scripts/Makefile.build:310: kernel/sched/build_policy.o] Error 1
make[1]: *** [scripts/Makefile.build:638: kernel/sched] Error 2
make[1]: *** Waiting for unfinished jobs....

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
[mcgrof: small build fix, we need sched_rt_can_attach() even
 when CONFIG_SYSCTL is disabled]
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>