GPU PCG node fails to cook if within Subgraph

When cooking a project that has a PCG graph with a subgraph containing a GPU node (Point Processor in the example provided), the cook fails with an illegal reference. However, the same GPU node used in the top level of a PCG graph cooks as expected.

Cook error:
```
UATHelper: Cooking (Windows): LogWindows: Error: appError called: Can't save 'C:/####/PCG_GPU_Subgraph/Saved/Cooked/Windows/PCG_GPU_Subgraph/Content/GPUSpawn.uasset': Illegal reference to private object: 'PCGNode /Game/GPUSubgraph.GPUSubgraph:CustomHLSL_0' referenced by 'PCGCustomKernelDataInterface_0' (at '/Game/GPUSpawn.GPUSpawn:GPUSpawn_1') in its 'Settings' property.
UATHelper: Cooking (Windows): LogWindows: Error: begin: stack for UAT
UATHelper: Cooking (Windows): LogWindows: Error: === Critical error: ===
UATHelper: Cooking (Windows): LogWindows: Error:
UATHelper: Cooking (Windows): LogWindows: Error: Can't save 'C:/####/PCG_GPU_Subgraph/Saved/Cooked/Windows/PCG_GPU_Subgraph/Content/GPUSpawn.uasset': Illegal reference to private object: 'PCGNode /Game/GPUSubgraph.GPUSubgraph:CustomHLSL_0' referenced by 'PCGCustomKernelDataInterface_0' (at '/Game/GPUSpawn.GPUSpawn:GPUSpawn_1') in its 'Settings' property.
UATHelper: Cooking (Windows): LogWindows: Error:
UATHelper: Cooking (Windows): LogWindows: Error:
UATHelper: Cooking (Windows): LogWindows: Error: end: stack for UAT
PackagingResults: Error: appError called: Can't save 'C:/####/PCG_GPU_Subgraph/Saved/Cooked/Windows/PCG_GPU_Subgraph/Content/GPUSpawn.uasset': Illegal reference to private object: 'PCGNode /Game/GPUSubgraph.GPUSubgraph:CustomHLSL_0' referenced by 'PCGCustomKernelDataInterface_0' (at '/Game/GPUSpawn.GPUSpawn:GPUSpawn_1') in its 'Settings' property.
PackagingResults: Error: begin: stack for UAT
PackagingResults: Error: === Critical error: ===
PackagingResults: Error: Can't save 'C:/####/PCG_GPU_Subgraph/Saved/Cooked/Windows/PCG_GPU_Subgraph/Content/GPUSpawn.uasset': Illegal reference to private object: 'PCGNode /Game/GPUSubgraph.GPUSubgraph:CustomHLSL_0' referenced by 'PCGCustomKernelDataInterface_0' (at '/Game/GPUSpawn.GPUSpawn:GPUSpawn_1') in its 'Settings' property.
PackagingResults: Error: end: stack for UAT
```

A simple repro project, `PCG_GPU_Subgraph`, is attached to this bug report.

1.) Load the provided `PCG_GPU_Subgraph` project.
2.) From the editor, start a cook operation.

Observe: The cook operation will fail with errors about an illegal reference.
Expected: The cook operation should not fail.