vk_command_pool: Add missing header guard

src/video_core/renderer_vulkan/vk_command_pool.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include <cstddef>
 #include <vector>