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.cpp

@@ -14,7 +14,7 @@ namespace Kernel {
 ReadableEvent::ReadableEvent(KernelCore& kernel) : WaitObject{kernel} {}
 ReadableEvent::~ReadableEvent() = default;
 
-bool ReadableEvent::ShouldWait(Thread* thread) const {
+bool ReadableEvent::ShouldWait(const Thread* thread) const {
     return !signaled;
 }