Fixed compile issue in unity builds.

Signed-off-by: santorac <55155825+santorac@users.noreply.github.com>

Gems/Atom/RPI/Code/Include/Atom/RPI.Edit/Material/MaterialTypeSourceData.h

@@ -22,6 +22,7 @@ namespace AZ
     namespace RPI
     {
         class MaterialTypeAsset;
+        class MaterialTypeAssetCreator;
         class MaterialFunctorSourceDataHolder;
         class JsonMaterialPropertySerializer;