sched/isolation: Make CONFIG_CPU_ISOLATION depend on CONFIG_SMP

kernel/sched/isolation.c obviously makes no sense without CONFIG_SMP, but
the Kconfig entry we have right now:

	config CPU_ISOLATION
		bool "CPU isolation"
		depends on SMP || COMPILE_TEST

allows the creation of pointless .config's which cause
build failures.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250330134955.GA7910@redhat.com

Closes: https://lore.kernel.org/oe-kbuild-all/202503260646.lrUqD3j5-lkp@intel.com/