cmake: allow using system VMA library

src/video_core/vulkan_common/vma.cpp

@@ -0,0 +1,8 @@
+// SPDX-FileCopyrightText: Copyright 2023 yuzu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#define VMA_IMPLEMENTATION
+#define VMA_STATIC_VULKAN_FUNCTIONS 0
+#define VMA_DYNAMIC_VULKAN_FUNCTIONS 1
+
+#include <vk_mem_alloc.h>