kernel/scheduler: Mark SchedulerLock constructor as nodiscard

Allows the compiler to warn about cases where the constructor is used
but then immediately discarded, which is a potential cause of
locking/unlocking bugs.

src/core/hle/kernel/scheduler.h

@@ -289,7 +289,7 @@ private:
 
 class SchedulerLock {
 public:
-    explicit SchedulerLock(KernelCore& kernel);
+    [[nodiscard]] explicit SchedulerLock(KernelCore& kernel);
     ~SchedulerLock();
 
 protected: