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/thread.cpp

@@ -28,7 +28,7 @@
 
 namespace Kernel {
 
-bool Thread::ShouldWait(Thread* thread) const {
+bool Thread::ShouldWait(const Thread* thread) const {
     return status != ThreadStatus::Dead;
 }