k_scheduler: Mark KScopedSchedulerLock as [[nodiscard]]

Prevents logic bugs like:

KScopedSchedulerLock{kernel};

instead of:

KScopedSchedulerLock lk{kernel};

from slipping through.

src/core/hle/kernel/k_scheduler.h

@@ -198,7 +198,7 @@ private:
     Common::SpinLock guard{};
 };
 
-class KScopedSchedulerLock : KScopedLock<GlobalSchedulerContext::LockType> {
+class [[nodiscard]] KScopedSchedulerLock : KScopedLock<GlobalSchedulerContext::LockType> {
 public:
     explicit KScopedSchedulerLock(KernelCore& kernel);
     ~KScopedSchedulerLock();