M_Volumetric_Sky_Prototype Material has compile errors

The material "M_Volumetric_Sky_Prototype" has numerous compile errors that prevent it from being used.

This was reported and tested in 4.25 Preview 2 (CL-12055273) and reproduced in 4.26 (CL-12192349)

1. Open the UE4 Editor and create a Blank project
2. Go to Plugins and enable "Volumetrics"
3. Show Engine and Plugin Content in the Content Browser
4. Navigate to Volumetrics Content/Content/Sky/Materials then open M_Volumetric_Sky_Prototype

Results: Numerous compile errors on the Material preventing it from rendering

Expected: The material is rendered and has no compilation errors