sched_ext: add CONFIG_DEBUG_INFO_BTF dependency

sched_ext: add CONFIG_DEBUG_INFO_BTF dependency

Without BTF, attempting to load any sched_ext scheduler will result in
an error like the following:

  libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?

This makes sched_ext pretty much unusable, so explicitly depend on
CONFIG_DEBUG_INFO_BTF to prevent these issues.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Tejun Heo <tj@kernel.org>