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>

kernel/Kconfig.preempt

@@ -135,7 +135,7 @@ config SCHED_CORE
 
 config SCHED_CLASS_EXT
 	bool "Extensible Scheduling Class"
-	depends on BPF_SYSCALL && BPF_JIT
+	depends on BPF_SYSCALL && BPF_JIT && DEBUG_INFO_BTF
 	help
 	  This option enables a new scheduler class sched_ext (SCX), which
 	  allows scheduling policies to be implemented as BPF programs to