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/fs/archive.cpp

@@ -108,7 +108,7 @@ void File::Read(Kernel::HLERequestContext& ctx) {
     std::chrono::nanoseconds read_timeout_ns{backend->GetReadDelayNs(length)};
     ctx.SleepClientThread(Kernel::GetCurrentThread(), "file::read", read_timeout_ns,
                           [](Kernel::SharedPtr<Kernel::Thread> thread,
-                             Kernel::HLERequestContext& ctx, ThreadWakeupReason reason) {
+                             Kernel::HLERequestContext& ctx, Kernel::ThreadWakeupReason reason) {
                               // Nothing to do here
                           });
 }