Fixed a link error on android (clang)

Gems/Atom/RPI/Code/Include/Atom/RPI.Reflect/Model/ModelMaterialSlot.h

@@ -26,7 +26,7 @@ namespace AZ
             static void Reflect(AZ::ReflectContext* context);
 
             using StableId = uint32_t;
-            static const StableId InvalidStableId = -1;
+            static const StableId InvalidStableId;
 
             //! This ID must have a consistent value when the asset is reprocessed by the asset pipeline, and must be unique within the ModelLodAsset.
             //! In practice, this set using the MaterialUid from SceneAPI. See ModelAssetBuilderComponent::CreateMesh.

Gems/Atom/RPI/Code/Source/RPI.Reflect/Model/ModelMaterialSlot.cpp

@@ -13,6 +13,10 @@ namespace AZ
 {
     namespace RPI
     {
+        // Normally this would be defined in the header file and substituted by the compiler, but for
+        // some reason clang doesn't accept it.
+        const ModelMaterialSlot::StableId ModelMaterialSlot::InvalidStableId = -1;
+
         void ModelMaterialSlot::Reflect(AZ::ReflectContext* context)
         {
             if (auto* serializeContext = azrtti_cast<AZ::SerializeContext*>(context))