kernel/wait_object: Make ShouldWait() take thread members by pointer-to-const

Given this is intended as a querying function, it doesn't make sense to
allow the implementer to modify the state of the given thread.

src/core/hle/kernel/readable_event.h

@@ -36,7 +36,7 @@ public:
         return HANDLE_TYPE;
     }
 
-    bool ShouldWait(Thread* thread) const override;
+    bool ShouldWait(const Thread* thread) const override;
     void Acquire(Thread* thread) override;
 
     /// Unconditionally clears the readable event's state.