graphics/vulkan-validation-layers: unbreak build with Clang 13

In file included from layers/generated/chassis.cpp:48:
In file included from layers/best_practices_validation.h:26:
In file included from layers/image_state.h:31:
In file included from layers/image_layout_map.h:30:
layers/subresource_adapter.h:67:5: error: definition of implicit copy assignment operator for 'Subresource' is deprecated because it has a user-declared copy constructor [-Werror,-Wdeprecated-copy]

Subresource(const Subresource& from) = default;
^

layers/subresource_adapter.h:305:42: note: in implicit copy assignment operator for 'subresource_adapter::Subresource' first required here

*static_cast<Subresource*>(this) = encoder_->Decode(index);
                                 ^

Reported by: antoine (via bug 258209 exp-run)