sched_ext: Guard cpu_smt_mask() with CONFIG_SCHED_SMT

sched_ext: Guard cpu_smt_mask() with CONFIG_SCHED_SMT

Wrap cpu_smt_mask() usage with CONFIG_SCHED_SMT to avoid build failures
on kernels built without SMT support.

Fixes: 2197cecdb02c ("sched_ext: idle: Prioritize idle SMT sibling")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603221422.XIueJOE9-lkp@intel.com/
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>