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/wait_object.h

@@ -24,7 +24,7 @@ public:
      * @param thread The thread about which we're deciding.
      * @return True if the current thread should wait due to this object being unavailable
      */
-    virtual bool ShouldWait(Thread* thread) const = 0;
+    virtual bool ShouldWait(const Thread* thread) const = 0;
 
     /// Acquire/lock the object for the specified thread if it is available
     virtual void Acquire(Thread* thread) = 0;