Fix vegetation test gem build on MSVC latest (#19086)

Signed-off-by: guillaume-haerinck <[email protected]>

Gems/Vegetation/Code/Tests/PrefabInstanceSpawnerTests.cpp

@@ -129,7 +129,7 @@ namespace UnitTest
         }
         AZ::Data::AssetHandler::LoadResult LoadAssetData(
             [[maybe_unused]] const AZ::Data::Asset<AZ::Data::AssetData>& asset,
-            AZStd::shared_ptr<AZ::Data::AssetDataStream> stream,
+            [[maybe_unused]] AZStd::shared_ptr<AZ::Data::AssetDataStream> stream,
             [[maybe_unused]] const AZ::Data::AssetFilterCB& assetLoadFilterCB) override
         {
             MockAssetData* temp = reinterpret_cast<MockAssetData*>(asset.GetData());