InfinityBaseWindow: Add missing header guard

Prevents potential inclusion issues.

Source/Core/DolphinQt/InfinityBase/InfinityBaseWindow.h

@@ -1,6 +1,8 @@
 // Copyright 2023 Dolphin Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+#pragma once
+
 #include <array>
 #include <string>