kernel/thread: Include thread-related enums within the kernel namespace

Previously, these were sitting outside of the Kernel namespace, which
doesn't really make sense, given they're related to the Thread class
which is within the Kernel namespace.

src/core/hle/service/vi/vi.cpp

@@ -514,7 +514,7 @@ private:
                 ctx.SleepClientThread(
                     Kernel::GetCurrentThread(), "IHOSBinderDriver::DequeueBuffer", -1,
                     [=](Kernel::SharedPtr<Kernel::Thread> thread, Kernel::HLERequestContext& ctx,
-                        ThreadWakeupReason reason) {
+                        Kernel::ThreadWakeupReason reason) {
                         // Repeat TransactParcel DequeueBuffer when a buffer is available
                         auto buffer_queue = nv_flinger->GetBufferQueue(id);
                         boost::optional<u32> slot = buffer_queue->DequeueBuffer(width, height);