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.cpp

@@ -247,7 +247,7 @@ void Process::Acquire(Thread* thread) {
     ASSERT_MSG(!ShouldWait(thread), "Object unavailable!");
 }
 
-bool Process::ShouldWait(Thread* thread) const {
+bool Process::ShouldWait(const Thread* thread) const {
     return !is_signaled;
 }