DolphinQt: Fix the panic alert deadlock, GPU thread edition

The fix in ef77872 worked for panic alerts from
the CPU thread, but there were still problems with
panic alerts from the GPU thread in dual core mode.
This change attempts to fix those.

Source/Core/Core/Core.h

@@ -98,6 +98,8 @@ void Shutdown();
 
 void DeclareAsCPUThread();
 void UndeclareAsCPUThread();
+void DeclareAsGPUThread();
+void UndeclareAsGPUThread();
 
 std::string StopMessage(bool main_thread, std::string_view message);