vulkan zcull: Fix deadlock in zcull flush waiting

Block adding additional flush requests until the first ones are treated (by adding missing lock)

rpcs3/Emu/RSX/VK/VKGSRender.cpp

@@ -3581,6 +3581,7 @@ void VKGSRender::get_occlusion_query_result(rsx::reports::occlusion_query_info*
 	{
 		if (data.command_buffer_to_wait == m_current_command_buffer)
 		{
+			std::lock_guard lock(m_flush_queue_mutex);
 			flush_command_queue();
 
 			//Clear any deferred flush requests from previous call to get_query_status()