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/process.h

@@ -237,7 +237,7 @@ private:
     ~Process() override;
 
     /// Checks if the specified thread should wait until this process is available.
-    bool ShouldWait(Thread* thread) const override;
+    bool ShouldWait(const Thread* thread) const override;
 
     /// Acquires/locks this process for the specified thread if it's available.
     void Acquire(Thread* thread) override;