sched_ext: Build fix on !CONFIG_STACKTRACE[_SUPPORT]

scx_dump_task() uses stack_trace_save_tsk() which is only available when
CONFIG_STACKTRACE. Make CONFIG_SCHED_CLASS_EXT select CONFIG_STACKTRACE if
the support is available and skip capturing stack trace if
!CONFIG_STACKTRACE.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407161844.reewQQrR-lkp@intel.com/


Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Tejun Heo <tj@kernel.org>