Fixed linker error specifically on windows

The error here is that when forward referencing a struct/class what matters is whether the entity actually is a struct or a class. For example, this is what the definitions look like if the entity is:
a class:  ?Init@SimpleDrawable@Scene@OpenVulkano@@QEAAXPEAVShader@23@PEAVGeometry@23@PEAVMaterial@23@@Z
a struct: ?Init@SimpleDrawable@Scene@OpenVulkano@@QEAAXPEAVShader@23@PEAVGeometry@23@PEAUMaterial@23@@Z

openVulkanoCpp/Scene/SimpleDrawable.hpp

@@ -11,7 +11,7 @@
 namespace OpenVulkano::Scene
 {
 	class Geometry;
-	class Material;
+	struct Material;
 
 	class SimpleDrawable final : public Drawable
 	{